Moving datadir completely breaks service startup on RPM based distros

The generated mysql.service file has the absolute path for the wsrep-start-position file hardcoded, and it turns out that path is hard coded to be the same as the default socket path (of all things).

This works out ok when moving/removing /var/lib/mysql on Debian based distributions where the socket file is located under /run, but on RPM distros it is hardcoded to /var/lib/mysql/wsrep_start_position; and so service startup breaks when removing the default datadir.

Even worse: it even fails on standalone servers that do not actually use Galera / wsrep at all.

Suggested fix:

• add RuntimeDirectory=mariadb to the service file
• use /run/mariadb/wsrep_start_position path everywhere

That way SystemD will take care of creating the directory under /run on service startup, and remove it again on service shutdown, so that it will always be present when needed