Customize dh_installinit to keep service enabled (Closes: #1029136)

As shown in https://salsa.debian.org/otto/mariadb-server/-/jobs/3900006 the mariadb service is no longer found after the package mariadb-server-10.6 has been purged. This Salsa-CI job was created to do exactly the scenario described in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029136.

When this fix is applied, the same step in job https://salsa.debian.org/otto/mariadb-server/-/jobs/3900125 passed.

There is also another approach tested with same test in https://salsa.debian.org/otto/mariadb-server/-/jobs/3902078 (on branch bugfix/1029136-empty-compat-packages9 which is using empty mariadb-server-10.6 package to try to solve it.

I don't think the latter solution is elegant - better to customize dh_installinit to solve this in maintainer scripts purely.

See commit message for details on what the code change is and why it is done.