mariadb: make it be able to work with SELinux

1. change the datadir from /var/mysql to /var/lib/mysql
2. after creating datedir, restorecon on it, make SELinux be able to
work
3. Add the PIDFile for systemd unit file, otherwise systemctl can not
stop mysqld

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>