default mysql -> mariadb

[nia <nia%NetBSD.org@localhost>]

Hello, I'm curious what people's thoughts are on making the default
MySQL implementation MariaDB. Obviously MySQL would still be avaialble
as an option. As far as I'm aware a few people are already using
MariaDB successfully on NetBSD, but I'd like to hear some more reports
in and we should get it working on SunOS too.

Any change would happen at the start of a new branch so we get
maximum feedback before a new stable branch is released.