* move rarely-used output and pty interfaces out of emu.h
* consolidate and de-duplicate forward declarations, also remove some obsolete ones
* clean up more #include guard macros
* scope down a few more things
(nw) Everyone, please keep forward declarations for src/emu in src/emu/emufwd.h -
this will make it far easier to keep them in sync with declarations than having
them scattered through all the other files.
* Eliminate globals/file statics
* Remove lots of stuff from global scope
* Use std::function for custom command registration
* Eliminate some trampolines
* Build fixes from Vas Crabb and balr0g
