mariadb: re-package configure files

There are 3 configure files under /etc/my.cnf.d:

client.cnf: options for client library
mysql-clients.cnf: options for MariaDB tools such as mysqladmin, etc
server.cnf: options for server

They are packaged to mariadb-leftover at this moment. That is not correct.
Split them into sub-packages:
client.cnf          --> libmysqlclient
mysql-clients.cnf   --> mariadb-client
server.cnf          --> mariadb-server

Make these files as configure file by CONFFILES and update global
configure file my.cnf to include /etc/my.cnf.d as well.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>