Extend mysql_instal_db to search plugins also from lib/*/mariadb19/plugin

In Debian official the MariaDB plugins (.so files) go to a separate
versioned directory so that upgrades and dependencies will work correctly.

This change extends this script to check the /usr/lib/*/mariadb19/..
directory in addition to what it already does. Without this change
the script fails with:

  $ mysql_install_db ...
  Cannot change ownership of the '/auth_pam_tool_dir' directory
  to the 'mysql' user. Check that you have the necessary permissions and
  try again.

scripts/mysql_install_db.sh

@@ -358,7 +358,7 @@ then
     cannot_find_file fill_help_tables.sql @pkgdata_locations@
     exit 1
   fi
-  plugindir=`find_in_dirs --dir auth_pam.so $basedir/lib*/plugin $basedir/lib*/mysql/plugin`
+  plugindir=`find_in_dirs --dir auth_pam.so $basedir/lib*/plugin $basedir/lib*/mysql/plugin $basedir/lib/*/mariadb19/plugin`
   pamtooldir=$plugindir
 # relative from where the script was run for a relocatable install
 elif test -n "$dirname0" -a -x "$rel_mysqld" -a ! "$rel_mysqld" -ef "@sbindir@/mysqld"