clean up extension disable flags
A thorough clean up of the extension disable flags
- unexport them - except for two that are used by drivers
- move the no*Extension fields away from OS layer to their corresponding extensions
- drop unnecessary includes of of extinit.h
- let all consumers consistently include exitinit_priv.h (which in turn includes extinit.h)
- move extinit_priv.h out of the global include dir (the public .h's home) to miext