There are now captions for specific types of music! (Currently signified with an M, but I'll change that in a bit...) A whole lotta things needed to be modified for this, though.

* Serious refactor of - and internal exposure of - what has become S_StartCaption().
* Renaming of a few existing captions.
* The prevention of access to - or writing over - sfx_None's stuff in SOC or Lua.
* The new Lua wrapper function S_StartMusicCaption(string, tics, optional player), which essentially allows custom music captions to be created. (This is best used for stuff like final lap music in SRB2 Kart or bonus time in a thokker-like game, not a comprehensive Now Playing expy. That'd be a different kettle of fish.)

Also, updated all the lock-on stuff to use P_IsLocalPlayer instead of rolling my own.