Partially reverts commit a4cc6fb9.

While all current versions of Linux have systemd, support for traditional
init.d is still needed e.g. on Linux subsystem on Windows, kFreeBSD and
special variants of Debian/Ubuntu that for other reasons don't have
systemd.

Thus, re-introduce the init file that was remove, but this time with
then name 'mariadb'.

Supporting traditional sysv init in paraller with systemd is easy, since
Debian has facilities for it.

Also simplify and update salsa-ci.yml install/upgrade testing works
for all previous MariaDB and MySQL releases without any excess quirks.

Note that in fresh installs the salsa-ci.yml needs to run command
'service mariadb status' to control the service, while on upgrades
it is enough to run 'service mysql status', since the init.d/mysql
file is left behind from previous install, along with some other
config files such as /etc/default/mysql and /etc/mysql/* stuff.