Having this feature documented in the mariadb postinst and
postrm scripts is needed so that the same packaging code
is easier to understand and re-use in downstream and upstream
packaging where the new mysql-common package might not
be available.

For the same reason, echo the mv command tips for users if
the packaging is used on a platform that does not have
the new mysql-common package.

Checking for the existance of the configure-symlink hook
on postrm is also useful for scenarios where the mysql-common
package gets purged before the mariadb-server-10.0 is purged.