(cherry picked from commit 9a5eb024)

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

(cherry picked from commit fdbe3e80)

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

(cherry picked from commit b330dc23)

* 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 76be77b9)

(cherry picked from commit cb4e0751)

(cherry picked from commit 0999b0f8)

(cherry picked from commit e72610a3)

(cherry picked from commit 8e05de17)

* Added reset flag to S_Start (now S_StartEx)

This reverts commit aa48278f.

This reverts commit 720a1212.

This reverts commit eafed06a.

This reverts commit 7b118550.