MDEV-4151 Mixed MySQL/MariaDB references in RPM upgrade error message

Update pre-install script to fix package detection and standardize
MariaDB/MySQL terminology in messages for improved upgrade reliability
and reduced user confusion.

All new code of the whole pull request, including one or several files that are
either new files or modified ones, are contributed under the BSD-new license. I
am contributing on behalf of my employer Amazon Web Services, Inc.

support-files/rpm/server-prein.sh

@@ -28,10 +28,11 @@ startup script in %{_sysconfdir}/init.d/.
 
   if [ "$old_family" != "$new_family" ]; then
     error_text="$error_text
-Upgrading directly from MySQL $old_family to MariaDB $new_family may not
-be safe in all cases.  A manual dump and restore using mysqldump is
-recommended.  It is important to review the MariaDB manual's Upgrading
-section for version-specific incompatibilities.
+Upgrading directly from MariaDB or MySQL $old_family
+to MariaDB $new_family may not be safe in all cases.
+A manual dump and restore using mariadb-dump (or mysqldump) is recommended.
+It is important to review the MariaDB manual's Upgrading section for
+version-specific incompatibilities.
 "
   fi
 
@@ -39,23 +40,23 @@ section for version-specific incompatibilities.
     cat <<HERE >&2
 
 ******************************************************************
-A MySQL or MariaDB server package ($installed) is installed.
+A MariaDB or MySQL server package ($installed) is installed.
 $error_text
 A manual upgrade is required.
 
 - Ensure that you have a complete, working backup of your data and my.cnf
   files
-- Shut down the MySQL server cleanly
-- Remove the existing MySQL packages.  Usually this command will
+- Shut down the MariaDB or MySQL server cleanly
+- Remove the existing MariaDB or MySQL packages. Usually this command will
   list the packages you should remove:
-  rpm -qa | grep -i '^mysql-'
+  rpm -qa | grep -iE '^(mariadb|mysql)-'
 
   You may choose to use 'rpm --nodeps -ev <package-name>' to remove
   the package which contains the mysqlclient shared library.  The
   library will be reinstalled by the MariaDB-shared package.
 - Install the new MariaDB packages supplied by $myvendor
 - Ensure that the MariaDB server is started
-- Run the 'mysql_upgrade' program
+- Run the 'mariadb-upgrade' program
 
 This is a brief description of the upgrade process.  Important details
 can be found in the MariaDB manual, in the Upgrading section.
@@ -65,8 +66,8 @@ HERE
   fi
 fi
 
-# Create a MySQL user and group. Do not report any problems if it already exists.
+# Create a MariaDB user and group. Do not report any problems if it already exists.
 groupadd -r %{mysqld_group} 2> /dev/null || true
-useradd -M -r --home %{mysqldatadir} --shell /sbin/nologin --comment "MySQL server" --gid %{mysqld_group} %{mysqld_user} 2> /dev/null || true
+useradd -M -r --home %{mysqldatadir} --shell /sbin/nologin --comment "MariaDB server" --gid %{mysqld_group} %{mysqld_user} 2> /dev/null || true
 # The user may already exist, make sure it has the proper group nevertheless (BUG#12823)
 usermod --gid %{mysqld_group} %{mysqld_user} 2> /dev/null || true