There are now captions for specific types of music! (Currently signified with...

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.