Add a toggle for loading audio from directories like "music"

The main purpose of this is letting you toggle the use of the Megaton Edition or World Tour's OGG files.

A few notes:

• You might be impacted by issue #211 (closed) while testing.
• This MR covers just Duke3D, but I added a CVAR outside of the duke3d tree.
• I wasn't sure where would be the right place for the CVAR and the variable that backs it. See the first commit.
• FX_SetPosition/MUSIC_SetSongPosition won't be called from the menu after toggling.
• "Use music dirs" might not be as clear as, say, "Use OGG tracks", but it should describe what the CVAR actually does for Duke3D.
• This MR wasn't tested at all with a USE_PHYSFS build. I don't know if this configuration is supported, either way.