Make OpenAL buffer management work on OS X

Don't assume we have unlimited OpenAL buffers.
Detach buffers from sources by setting AL_BUFFER to 0. Cannot delete buffers on OS X immediately after alSourceUnqueueBuffers.
Free unprocessed stream and music buffers.
Free unused sfx buffers when sound file exists and fails loading into OpenAL (though I haven't seen it fail).
Fix restoring loop sound position on OS X

The default OpenAL library on OS X errors AL_INVALID_VALUE for AL_SEC_OFFSET if source isn't playing.
Fix cinematics often being silent on OS X

If OpenAL source isn't playing, often what happens is: queued buffers is 0 and processed buffers is non-0.
S_AL_StreamUpdate won't play the steam if queued buffers 0.
So start the stream in S_AL_RawSamples.
Remove unused cvar_t variable cl_trn
Fix comparing unsigned values to < 0 in gl2
Merge remote-tracking branch 'ioquake3/master'
Fix comparing unsigned value to < 0 in gl2 for foliage