* moved the sprite renaming out of the file system entirely into a caller-provided callback.
* renamed several functions to closer match the terms of a file system.
* moved the VM interface out of the implementation.
The main exits are initiated from code that cannot filter this back to D_DoomMain easily so the exception is the only way to get there.
The 3 main points of exit are:
* quit/exit CCMD
* quitting the menu through ST_Endoom
* receiving a quit message on the main window.