MDEV-30952: Fix shellcheck problems on Debian scripts

Commit fixes several ShellCheck found problems in Debian
Pre- and Postscripts.

Debian script mariadb-server-10.6.postrm contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2166 -- Prefer [ p ] && [ q ] as [ p -a q...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...

Debian script mariadb-server-10.6.postinst contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2129 -- Consider using { cmd1; cmd2; } >>...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...
  https://www.shellcheck.net/wiki/SC1072 -- Expected test to end here (don't ...
  https://www.shellcheck.net/wiki/SC1073 -- Couldn't parse this test expressi...
  https://www.shellcheck.net/wiki/SC1009 -- The mentioned syntax error was in...

Debian script mariadb-server-10.6.preinst contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2231 -- Quote expansions in this for loop...
  https://www.shellcheck.net/wiki/SC2166 -- Prefer [ p ] && [ q ] as [ p -a q...
  https://www.shellcheck.net/wiki/SC2001 -- See if you can use ${variable//se...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...
  https://www.shellcheck.net/wiki/SC1007 -- Remove space after = if trying to...

debian/mariadb-server-10.6.postinst

 #!/bin/bash
 set -e
 
+# shellcheck source=/dev/null
 . /usr/share/debconf/confmodule
 
 # Automatically set version to ease maintenance of this file
@@ -112,8 +113,10 @@ EOF
     # This direct update is needed to enable an authentication mechanism to
     # perform mariadb-upgrade, (MDEV-22678).  To keep the impact minimal, we
     # skip innodb and set key-buffer-size to 0 as it isn't reused.
-    if [ -f "$mysql_datadir"/auto.cnf ] && [ -f "$mysql_datadir"/mysql/user.MYD ] &&
-    [ ! lsof -nt "$mysql_datadir"/mysql/user.MYD > /dev/null ] && [ ! -f "$mysql_datadir"/undo_001 ]; then
+    lsof -nt "$mysql_datadir"/mysql/user.MYD > /dev/null
+    lsof_rtn_code=$?
+    if [ -f "$mysql_datadir/auto.cnf" ] && [ -f "$mysql_datadir/mysql/user.MYD" ] &&
+    [ ! ${lsof_rtn_code} ] && [ ! -f "$mysql_datadir/undo_001" ]; then
       echo "UPDATE mysql.user SET plugin='unix_socket' WHERE plugin='auth_socket';" |
       mariadbd --skip-innodb --key_buffer_size=0  --default-storage-engine=MyISAM --bootstrap 2> /dev/null
     fi
@@ -189,20 +192,22 @@ EOF
     fi
     if [ ! -e "$dc" ]; then
       cat /dev/null > $dc
-      echo "# THIS FILE IS OBSOLETE. STOP USING IT IF POSSIBLE."       >>$dc
-      echo "# This file exists only for backwards compatibility for"   >>$dc
-      echo "# tools that run '--defaults-file=/etc/mysql/debian.cnf'"  >>$dc
-      echo "# and have root level access to the local filesystem."     >>$dc
-      echo "# With those permissions one can run 'mariadb' directly"   >>$dc
-      echo "# anyway thanks to unix socket authentication and hence"   >>$dc
-      echo "# this file is useless. See package README for more info." >>$dc
-      echo "[client]"                                                  >>$dc
-      echo "host     = localhost"                                      >>$dc
-      echo "user     = root"                                           >>$dc
-      echo "[mysql_upgrade]"                                           >>$dc
-      echo "host     = localhost"                                      >>$dc
-      echo "user     = root"                                           >>$dc
-      echo "# THIS FILE WILL BE REMOVED IN A FUTURE DEBIAN RELEASE."   >>$dc
+      {
+        echo "# THIS FILE IS OBSOLETE. STOP USING IT IF POSSIBLE.";
+        echo "# This file exists only for backwards compatibility for";
+        echo "# tools that run '--defaults-file=/etc/mysql/debian.cnf'";
+        echo "# and have root level access to the local filesystem.";
+        echo "# With those permissions one can run 'mariadb' directly";
+        echo "# anyway thanks to unix socket authentication and hence";
+        echo "# this file is useless. See package README for more info.";
+        echo "[client]";
+        echo "host     = localhost";
+        echo "user     = root";
+        echo "[mysql_upgrade]";
+        echo "host     = localhost";
+        echo "user     = root";
+        echo "# THIS FILE WILL BE REMOVED IN A FUTURE DEBIAN RELEASE.";
+      } >> $dc
     fi
     # Keep it only root-readable, as it always was
     chown 0:0 $dc

debian/mariadb-server-10.6.postrm

 #!/bin/bash
 set -e
 
+# shellcheck source=/dev/null
 . /usr/share/debconf/confmodule
 
 # Automatically set version to ease maintenance of this file
@@ -26,7 +27,7 @@ stop_server() {
   set -e
 
   # systemctl could emit exit code 100=no init script (fresh install)
-  if [ "$errno" != 0 -a "$errno" != 100 ]; then
+  if [ "$errno" != 0 ] && [ "$errno" != 100 ]; then
     echo "Attempt to stop MariaDB/MySQL server returned exitcode $errno" 1>&2
     echo "There is a MariaDB/MySQL server running, but we failed in our attempts to stop it." 1>&2
     echo "Stop it yourself and try again!" 1>&2
@@ -38,7 +39,7 @@ stop_server() {
 
 case "$1" in
   purge|remove|upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)
-    if [ -n "`$MYADMIN ping 2>/dev/null`" ]; then
+    if [ -n "$($MYADMIN ping 2>/dev/null)" ]; then
       stop_server
       sleep 2
     fi

debian/mariadb-server-10.6.preinst

@@ -7,6 +7,7 @@
 #        * <old-preinst> abort-upgrade <new-version>
 #
 
+# shellcheck source=/dev/null
 . /usr/share/debconf/confmodule
 
 # Automatically set version to ease maintenance of this file
@@ -40,7 +41,7 @@ stop_server() {
   set -e
 
   # systemctl could emit exit code 100=no init script (fresh install)
-  if [ "$errno" != 0 -a "$errno" != 100 ]; then
+  if [ "$errno" != 0 ] && [ "$errno" != 100 ]; then
     echo "Attempt to stop MariaDB/MySQL server returned exitcode $errno" 1>&2
     echo "There is a MariaDB/MySQL server running, but we failed in our attempts to stop it." 1>&2
     echo "Stop it yourself and try again!" 1>&2
@@ -57,7 +58,7 @@ max_upgradeable_version=5.7
 # Check if a flag file is found that indicates a previous MariaDB or MySQL
 # version was installed. If multiple flags are found, check which one was
 # the biggest version number.
-for flag in $mysql_datadir/debian-*.flag
+for flag in "$mysql_datadir"/debian-*.flag
 do
 
   # The for loop leaves $flag as the query string if there are no results,
@@ -92,7 +93,7 @@ done
 # Downgrade is detected if the flag version is bigger than $this_version
 # (e.g. 10.1 > 10.0) or the flag version is smaller than 10.0 but bigger
 # than $max_upgradeable_version.
-if [ ! -z "$found_version" ]
+if [ -n "$found_version" ]
 then
 
   # MySQL 8.0 in Ubuntu has a bug in packaging and the file is name wrongly
@@ -134,7 +135,7 @@ fi
 
 # Don't abort dpkg if downgrade is detected (as was done previously).
 # Instead simply move the old datadir and create a new for this_version.
-if [ ! -z "$downgrade_detected" ]
+if [ -n "$downgrade_detected" ]
 then
   db_input critical "mariadb-server-$MAJOR_VER/old_data_directory_saved" || true
   db_go