MDEV-15526 SysV init service deployed file '/etc/init.d/mysql' prevents systemctl disable command to work correctly (mariadb|mysql naming support) (debian/ubuntu)

mariadb service is controlled by systemd

Do not install SysV init script on Debian/Ubuntu in /etc/init.d/
put it in /usr/share/mysql in case the user would need it for something.

Use systemctl in pre/post scripts directly
without funky conditionals and wrapper scripts