mariadb: improve packaging and configuration

mariadb: improve packaging and configuration

At present there are some flaws related to configuration, also related
to the packaging of mariadb. For starters there are complaints that the
configuration is too static.

To address this a new configuration layout is introduced. The primary
configuration file (my.cnf) is changed so that it now only includes
further configuration files in the directory /etc/mysql/conf.d. More
default configuration files are added for the server and the client.
This is the new default configuration.

With these changes it's possible for the user to select if they want to
change the default configuration (in conf.d/*.cnf) or if they want to
drop their own files into conf.d instead. If the user .cnf files are
read after the default .cnf files (files are included in alphabetical
order), they will overwrite the settings from the default configuration.

The other flaw is that the my.cnf file is included in mariadb-server.
But that doesn't really fit the requirements, as the client also uses
the configuration file(s). To accomodate this a new package
mariadb-common is added. It installs the shared my.cnf file.

The remaining changes add base packages, both for the server and the
client. These are meant as foundation for the packages containing the
respective binaries. In summary they will install the configuration,
small miscellaneous files (SQL scripts etc.) and the user "mariadb".

That means that everything is ready for the binaries, like mysql and
mysqld. If there is not enough space left on flash memory, the user can
just drop the binaries on a pendrive, link them to /usr/bin and get
started.

The ideas and configuration files were copied from Debian. Some
amendments were made.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>