I've voided this out on other sound interfaces than SDL Mixer ones because I'm both not sure whether they need it, and not sure how to make them work with it if they do.

(cherry picked from commit 9e6eebeb8d6b4119d87e9678bb6b0687e3dc5217)

(cherry picked from commit cf065e106f68fb8af3b0568eba188c8235b1656e)

(cherry picked from commit d5ec38815968e267aceb59a48a6cb6d3292c0b69)

(cherry picked from commit 9fb9386f84b3739fe765a78bcb9683eb7e98bc36)

(cherry picked from commit 14b393ab16736bb44dab6fd4a90b7bdd8ff782e0)

(cherry picked from commit 9a5fc5f66a31baf713c6f32f7a696b933da6907b)

This reverts commit 58fbb463.

# Conflicts:
#	src/android/i_sound.c
#	src/dummy/i_sound.c
#	src/win32/win_snd.c

This reverts commit 8b46bce1.

* music_lumpnum, music_data, music_handle are gone
* Relevant interfaces I_PlaySong are transferred to I_StartDigSong

# Conflicts:
#	src/lua_baselib.c
#	src/nds/i_sound.c
#	src/sdl12/mixer_sound.c
#	src/sdl12/sdl_sound.c
#	src/win32ce/win_snd.c

# Conflicts:
#	src/nds/i_sound.c
#	src/sdl12/sdl_sound.c
#	src/win32ce/win_snd.c