* change the license of the main file because there is no more id-based code here licensed under the GPL.
* moved VM interface definition out of the implementation.
* moved idmus CCMD out of implementation because it is dependent on Doom level definitions.
* moved s_music.cpp into the music folder with the rest of the music code.
There were two booleans that could be merged into the flag word.
This also fixes a bug with CHAN_NOSTOP not working for local sounds because it checked the wrong sound source for the playing sound.
Notable changes:
* IsSourcePlayingSomething had undefined behavior when checking unattached and unpositioned sounds.
* loading Blood RAW sounds with different sample rates but the same backing lump may not use the same hardware sound buffer.
* when playing an unpositioned sound the attenuation is irrelevant and must be ignored. This resulted in a 3D sound being started which was mostly inaudible due to lack of valid origin.
- added Marisa Kirisame's CHAN_OVERLAP flag.
- exported S_IsActorPlayingSomething to ZScript.
The sound API change required deprecating A_PlaySound and S_Sound. There are now new variants S_StartSound and A_StartSound which have two distinct parameters for channel and flags.
Many of the simple wrappers have been moved to a separate file and the sound source handling has been abstracted.
This is only the first phase, the work is not complete yet.
Also changed the license of the sound code to BSD after verifying that this code bears no similarity to id's original sound code anymore, save for a few function names (which are due to be refactored out anyway.)
