dev-perl/DBD-mysql: Add mysql/mariadb switching

dev-perl/DBD-mysql: Add mysql/mariadb switching

This was tangentially supported back with the virtual/mysqlclient stuff
and this attempts to bring this logic back.

However, in testing, this release currently failed to compile with
USE="mariadb -mysql", as DBD-mysql supporting mariadb is merely "best
effort", not a primary design goal, so can often be a bit behind the
curve.

Future releases may however work fine.

Though its recommended if you work with MariaDB to:
a. use DBD-MariaDB instead
b. use "mysql -mariadb" to use the mysql-connector-c implementation
   and then use that to connect to mariadb, which *may* work better.

As such, this flag is primarily to give user choice, but will be
use.stable.masked' until we can expect it to be reliable

Keywords dropped:
- alpha ( due to mariadb-connector-c )

Package-Manager: Portage-2.3.72, Repoman-2.3.16
Signed-off-by: Kent Fredric <kentnl@gentoo.org>