Lots of minor fixes and improvements to the sound engine, plus a few more important...

Lots of minor fixes and improvements to the sound engine, plus a few more important changes:
- moved the internal structures and API into a separate file (snd_main.h)
- added a lock mecanism on SFXs to free them as soon as they're no longer used
- simplified the sound API
- minor fixes and comment changes here and there
- independent sound volume for fake CD tracks
- updated MSVC and Dev-C++ project files accordingly
- removed unused functions
- added a SFX flag to make sure the engine complains at most one time for each missing sound file
- made the SFX list a linked list to remove the hardcoded limit for the number of SFX a client can handle
- changed the way ambient sounds are managed to be more consistent with the new API
- factorized some code
- removed the useless IN_Accumulate in the Win32 specific code
- removed the table for 8bit sound mixing (snd_scaletable)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4624 d7cf8633-e32d-0410-b094-e92efae38249