Prefer the systemctl edit mysql.service syntax
and leave the more complex alternatives to the
existing documents referenced.
Also show how to use the multiinstance a bit more.
Cherry-picked from Fedora: a87e9e5d9a
Related: #1971248
MariaDB-10.4 onwards included a pam_helper subprocess to help
with the pam authentication module.
If the user is running with Galera there are SST modules that could
be executing.
By dropping KillMode=process this reverts back to control-group to
cover all of these subprocesses. This is what upstream does.
https://jira.mariadb.org/browse/MDEV-25233 suggests moving to
KillMode=mixed, which is probably ok too, but has been tested less.
Cherry-picked from Fedora: 95f558b833
Related: #1971248
