databases/p5-DBD-mysql: Fix for MariaDB users

Add a new variable DBD_MYSQL, use that in *_DEPENDS and add mysql to USES
where required. DBD_MYSQL will automatically set the correct DBD dependency.

In cd16748194e2 databases/p5-DBD-mysql was updated to 5.x, in 5.x the
support for MariaDB was removed and only MySQL >= 8.0 is supported.

In the 4.x releases MariaDB is still supported, according to upstream 4.x
will still be supported for a while [1], so use that for now when we detect
that MYSQL_FLAVOUR is set to mariadb. databases/p5-DBD-mysql4 was added in
d95f49cb3b54.

DBD:MariaDB would be another alternative, but migrating to it might need
more analysis than just staying with the 4.x releases of p5-DBD-mysql. This
thread on the amavis-users mailing list has a very good summary [2] from
the author of DBD:MariaDB.

This includes ideas and suggestions from vvd and mat. Thanks!

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277889#c17
[2] https://lists.amavis.org/pipermail/amavis-users/2024-March/006823.html

PR: 275100, 277889
Reviewed by: vvd, mat
Fixes: cd16748194e2
Differential Revision: https://reviews.freebsd.org/D44662