doom music and opl3 emulation

[qwx <qwx@sciops.net>]

Hello,

I've been working on a MIDI player which supports GENMIDI lumps in doom wad
files.  These are instrument banks used with OPL2-compatible sound cards.

The end result is a fairly good approximation of what doom sounds like on DOS
with such a sound card.

To try it out:

- grab dmid, opl3 and rc/dmus from my contrib
- install the dmus script so it's accessible as /bin/dmus
- cd dmid; mk dmid.install
- cd opl3; mk install
- mount the doom wad you'll play using wadfs, for example:
	games/wadfs /sys/games/lib/doom/doom2.wad
- launch games/doom with the same wad

Wadfs is required to access the wad's GENMIDI (it will be accessible as
/mnt/wad/genmidi by default).  Dmid uses GENMIDI to convert a MIDI file into
OPL3 instructions;  opl3 then synthesizes pcm audio.  If the dmus script
exists, it is launched by doom to play a music lump, and it writes raw
audio to its stdout.

I'm not sure these belong in 9front, since all three components are required,
it's a bunch of code, and they don't yet have a use outside of doom music.

Anyway, enjoy,

qwx