Robert Collins' Cygwin file mode support and ufs writecleanswap bugfix.

Robert Collins' Cygwin file mode support and ufs writecleanswap bugfix.
Without it users on cygwin with text mode mounts will die badly,
and all cygwin users will not ever see a (CLEAN) status on their
cache_dir's. Why I was getting clean status when I first looked
into this may be due to some of the 'emulation' work done on cygwin
to make UN*X assumptions (ie unlinking open files) work somewhat.