mariadb: Add switch for innodb_snapshot_isolation

With the 11.6.2 release mariadb has made snapshot isolation a default
setting which recently caused issues on the AUR and the Forums, where
the respective database engines would fail due to the table changing
since the last time they read it:

    PHP Fatal error:  Uncaught mysqli_sql_exception: Record has changed since last read in table 'fluxbb_online' in /srv/http/fluxbb/include/dblayer/mysqli_innodb.php:79
    Stack trace:
    #0 /srv/http/fluxbb/include/dblayer/mysqli_innodb.php(79): mysqli_query()
    #1 /srv/http/fluxbb/include/functions.php(485): DBLayer->query()
    #2 /srv/http/fluxbb/include/common.php(162): update_users_online()
    #3 /srv/http/fluxbb/viewtopic.php(10): require('...')
    #4 {main}

We therefore introduce a config switch to restore the old behavior and
apply the changed setting on the two services.

Link: https://mariadb.com/kb/en/mariadb-11-6-2-release-notes/#innodb
Link: https://bbs.archlinux.org/viewtopic.php?id=301802
Link: aurweb#525


Signed-off-by: Christian Heusel <christian@heusel.eu>