Because musicplus-libopenmpt deleted it, I added a cautionary note to ALWAYS clean up your vars!

(cherry picked from commit 5b724e18b5268b0492b3f8b8980aadd2dd41e235)

* Check SDL Mixer 2.0.3 for MUS_MODPLUG_UNUSED, MUS_MP3_MAD_UNUSED
* Mixed D+C in I_LoadSong

* Mixer: make I_StartupSound return early if already set up
* Restartaudio: Add StopSFX call

* Mixer code has been in mixer_sound.c; this file is not invoked unless compiling with NOMIXER=1
* Remove everything under #ifdef HAVE_MIXER because this is never triggered
* Comment out #ifdef HAVE_LIBGME because we don't support playing music anyway (but theoretically, it could have worked separately from Mixer)
* Stub new music calls

(cherry picked from commit a7ae0599)

(cherry picked from commit e88d1477)

* Stop-gap for now. Ideally the logic would be in the respective places.

# Conflicts:
#	src/sdl/mixer_sound.c

(cherry picked from commit eae5d333)

# Conflicts:
#	src/sdl/mixer_sound.c

(cherry picked from commit d39b7011)

(cherry picked from commit fb6f8c8a0752c229f5b7369729437c8fecb54d45)