Experimental MikMod support
This patch adds experimental MikMod support which could possibly be used to play the MOD music from the Amiga versions of some games. Originally, I had planned to support both MikMod and ModPlug, but I decided MikMod was probably much more common.
This code is not particularly well tested, and it does not support seeking. But it's a start.
Ticket imported from: #1372271. Ticket imported from: patches/596.