audio/cmus: Disable MAD by default

According to the submitter libmad is vulnerable and inefficient and
should not be used anymore. The ffmpeg plugin serves as a fallback
input plugin. This should not cause any functional differences
other than slightly slower seek in MP3s.

Submitted by: daniel.engberg.lists_pyret.net
Differential Revision: https://reviews.freebsd.org/D22264