MDEV-6284: Sync correct Debian handling of auth_pam_tool

Manages the security risk in way that also fixes Lintian warning:
  W: mariadb-server-10.5: setuid-binary
  usr/lib/mysql/plugin/auth_pam_tool_dir/auth_pam_tool 4755 root/root

Applied downstream in
https://salsa.debian.org/mariadb-team/mariadb-10.4/-/commit/9605a48a99a61f439698676da192799de45d732a

debian/mariadb-server-10.5.postinst

@@ -95,7 +95,23 @@ EOF
     chmod 2750 $mysql_logdir
     set -e
 
-    # Set the correct filesystem ownership for the PAM v2 plugin
+    ## Set the correct filesystem ownership for the PAM v2 plugin
+    # eg. /usr/lib/x86_64-linux-gnu/mysql/plugin/auth_pam_tool_dir/
+    # NOTE! This is security sensitive, don't allow for a race condition.
+    #
+    # 1. Drop privileges of directory
+    # -> At this point only root can see and execute auth_pam_tool
+    chmod 0700 /usr/lib/mysql/plugin/auth_pam_tool_dir
+    #
+    # 2. Make binary setuid
+    # -> At this point only root can run the setuid binary so no escalation here yet
+    chmod 04755 /usr/lib/mysql/plugin/auth_pam_tool_dir/auth_pam_tool
+    #
+    # 3. Allow user 'mysql' to see and execute auth_pam_tool
+    # -> Now user mysql owns the directory and can see and execute the binary inside
+    # -> Since the binary is setuid, user mysql gets limited root powers here to
+    #    run the PAM authetications, which need root (e.g. to validate passwords
+    #    against /etc/shadow)
     chown mysql /usr/lib/mysql/plugin/auth_pam_tool_dir
 
     # This is important to avoid dataloss when there is a removed

debian/rules

@@ -164,11 +164,6 @@ endif
 	# Move test plugins that are only needed by the client to the libmariadb path
 	mv -v $(TMP)/usr/lib/mysql/plugin/qa_auth_client.so $(TMP)/usr/lib/$(DEB_HOST_MULTIARCH)/libmariadb3/plugin/
 
-override_dh_fixperms:
-	dh_fixperms
-	chmod 04755 debian/mariadb-server-10.5/usr/lib/mysql/plugin/auth_pam_tool_dir/auth_pam_tool
-	chmod  0700 debian/mariadb-server-10.5/usr/lib/mysql/plugin/auth_pam_tool_dir
-
 override_dh_installlogrotate-arch:
 	dh_installlogrotate --name mysql-server