New systemd socket activation can cause MariaDB to listen on a different TCP port than the one explicitly configured by the administrator

In https://github.com/MariaDB/server/commit/9fa3f51f59f5b7a6d84c68a45e6a6579f55847b3 (MDEV-38313) the `mariadb.socket` systemd files got these added:

[Install]

WantedBy=sockets.target

In https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1141976 it was reported that this change can lead to MariaDB systems to listen to `0.0.0.0:3306` on systems that were manually configured to only listen to localhost at `127.0.0.1:3306`. This is because the `mariadb.socket` systemd files were previously inert and not active, but with this change some undocumented thing in dh_installsystemd makes it activate the socket files.

I am still investigating the details but wanted to file this upstream for tracking, and in case other distros or the packages from MariaDB.org itself might also be affected by this regression.