use environment file in systemd units for _WSREP_START_POSITION

We used to run `systemctl set-environment` to pass
_WSREP_START_POSITION. This is bad because:

* it clutter systemd's environment (yes, pid 1)
* it requires root privileges
* options (like LimitNOFILE=) are not applied

Let's just create an environment file in ExecStartPre=, that is read
before ExecStart= kicks in. We have _WSREP_START_POSITION around for the
main process without any downsides.

support-files/CMakeLists.txt

@@ -32,6 +32,8 @@ ELSE()
   SET(MYSQLD_GROUP "mysql")
   SET(ini_file_extension "cnf")
   SET(HOSTNAME "uname -n")
+  get_filename_component(MYSQL_UNIX_DIR ${MYSQL_UNIX_ADDR} DIRECTORY)
+  SET(mysqlunixdir ${MYSQL_UNIX_DIR})
 ENDIF()
 
 # XXX: shouldn't we just have variables for all this stuff and centralise

support-files/mariadb.service.in

@@ -70,20 +70,20 @@ ProtectSystem=full
 # Prevent accessing /home, /root and /run/user
 ProtectHome=true
 
-# Execute pre and post scripts as root, otherwise it does it as User=
-PermissionsStartOnly=true
+# Use an environment file to pass variable _WSREP_START_POSITION
+EnvironmentFile=-@mysqlunixdir@/wsrep-start-position
 
 @SYSTEMD_EXECSTARTPRE@
 
 # Perform automatic wsrep recovery. When server is started without wsrep,
 # galera_recovery simply returns an empty string. In any case, however,
 # the script is not expected to return with a non-zero status.
-# It is always safe to unset _WSREP_START_POSITION environment variable.
+# It is always safe to remove @mysqlunixdir@/wsrep-start-position
+# environment file.
 # Do not panic if galera_recovery script is not available. (MDEV-10538)
-ExecStartPre=/bin/sh -c "systemctl unset-environment _WSREP_START_POSITION"
 ExecStartPre=/bin/sh -c "[ ! -e @bindir@/galera_recovery ] && VAR= || \
  VAR=`cd @bindir@/..; @bindir@/galera_recovery`; [ $? -eq 0 ] \
- && systemctl set-environment _WSREP_START_POSITION=$VAR || exit 1"
+ && echo _WSREP_START_POSITION=$VAR > @mysqlunixdir@/wsrep-start-position || exit 1"
 
 # Needed to create system tables etc.
 # ExecStartPre=@scriptdir@/mysql_install_db -u mysql
@@ -96,7 +96,7 @@ ExecStartPre=/bin/sh -c "[ ! -e @bindir@/galera_recovery ] && VAR= || \
 ExecStart=@sbindir@/mysqld $MYSQLD_OPTS $_WSREP_NEW_CLUSTER $_WSREP_START_POSITION
 
 # Unset _WSREP_START_POSITION environment variable.
-ExecStartPost=/bin/sh -c "systemctl unset-environment _WSREP_START_POSITION"
+ExecStartPost=/bin/rm -f @mysqlunixdir@/wsrep-start-position
 
 @SYSTEMD_EXECSTARTPOST@