Windows (MinGW): Do crash dumps, same as on *nix.

We do this by defining USE_UNIX_SIGNALS there and fixing the resulting
compiler errors.

For some reason they aren't being written to files yet, though :-(.

Conflicts:
	crawl-ref/source/AppHdr.h
	crawl-ref/source/crash-u.cc
	crawl-ref/source/main.cc