Split the big my.cnf into multiple smaller files with the same filenames
and contents as official Debian/Ubuntu packaging has.

The config contents stays the same apart from following additions
which the original MariaDB upstream configs had and probably needs
to be kept:
- lc-messages=en_US and skip-external-locking in server config

Configs the original MariaDB upstream had that are seemingly
unnecessary and thus removed:
- port=3306 removed from the client config
- log_warnings=2 removed from server config

Also adopt update-alternatives system using
mysql-common/configure-symlinks. This way it is aligned with
downstream Debian/Ubuntu packaging.