Add --log-class option, improve logging classes.

Add --log-class option, improve logging classes.
Replace SG_GENERAL by more specific log classes in many places.
Allow "," to separate logging classes (using "|" is odd on the
command-line).
Also add new option to make logging more useful for developers.
You can use:
  --log-level=debug --log-class=environment
to only get environment debug messages, or
  --log-level=debug --log-class=sound,ai
to only get debug messages related to the sound or AI subsystem.