• Added new default unset option MYSQL

If mysql is used instead of sqlite, teamspeak can fail to start because
mysql sometimes starts after teamspeak. As a fix, a new option has been
added that will add mysql to REQUIRES and set mysql server as a depend.

Reported by: Dries Michiels <driesmp@hotmail.com>
Reviewed by: lifanov (mentor)
Approved by: lifanov (mentor)
Differential Revision: https://reviews.freebsd.org/D11681