integrating latest mingw headers/startup

[Mumit Khan]

I had forgotten that the mingw headers/startup code in winsup is somewhat
outdated. I have the latest snapshot, included in mingw32 egcs-1.1.2 bin
release, on my ftp site:
  ftp://ftp.xraylith.wisc.edu/pub/khan/gnu-win32/mingw32/runtime/

Please let me know if I need to do something to get these into the Cygwin
snapshots. It's probably easier to import than to do diffs, but if diffs
are needed instead, I can do that as well against Cygwin winsup tree. 

The major changes include header fixes (eg., deleting deprecated alloc.h,
deprecate dir.h but keeping for compatibility, signal header fixes, and
most importantly fixes to startup code to handle signals correctly). The
sources are based on Colin Peters' latest 980701 snapshot + local fixes.

Regards,
Mumit