spearmint: Add internal support for multiple streaming sounds
spearmint: Free AL streaming sound buffers

Store buffers so they are freed in S_AL_FreeStreamChannel.
spearmint: Make Base increase streaming sound play count even if not looping

Match OpenAL code.
spearmint: Restore AL initial buffering for streaming sound

Do the initial read when starting the track, fixes gap/studder.
Not sure why I removed it.
spearmint: Make S_AL_StopBackgroundTrack use stream die function
spearmint: Add internal interface for streaming sounds
spearmint: Expose streaming sound API to cgame
spearmint: Fix StartBackgroundTrack syscall loop volume argument

Before volume was used for loop volume too.
Also move StopBackgroundTrack to be with sound syscalls.
spearmint: Fix attaching AL streaming sound to entity

It was staying full volume, now it uses S_AL_ScaleGain to fade out.
spearmint: Fix StartBackgroundTrack to loop intro by default