Add miscfuncs.h to files as needed throughout.

Add miscfuncs.h to files as needed throughout.
* mount.cc: New file.
* path.cc: Move mount-specific stuff into mount.cc.  Move common stuff into
miscfuncs.cc.  Remove unneeded includes.
* miscfuncs.cc: Move some common path functions here.
* miscfuncs.h: New file.
* winsup.h: Move miscelleneous functions to miscfuncs.h.
* dcrt0.cc: Remove unneeded includes.
* Makefile.in (DLL_OFILES): Add mount.o.
* include/cygwin/config.h: Fix a minor typo.