audio/xmms2: Default to using libmpg123 instead of libmad

[daniel.engberg.lists_pyret.net]

Summary

libmpg123 is much faster than libmad and is also in active development compared to libmad which hasn't seen any upstream development in years.

[madpilot]

Sorry for the delay.

I have no objection but please allow me a little more time to test the change before approving it.