Fake CD tracks support; DP now tries to play "sound/cdtracks/trackXX.wav" instead...

Fake CD tracks support; DP now tries to play "sound/cdtracks/trackXX.wav" instead of CD track XX. If it fails, it tries "trackXX.ogg", and then the real CD track if any. CD console commands should work as expected.
"cd_shared.c" is now part of the common files so it allows people to compile DP without real CD support, but with fake CD tracks support. "cd_null.c" is now a null driver, at the same level as "cd_linux.c" or "cd_win.c".
Fixed the broken return value of S_StartSound and a potential memory leak in S_StopAllSounds in the process.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4085 d7cf8633-e32d-0410-b094-e92efae38249