* cygwin:
Assorted cleanups to Luke's cygwin fixes.
Makefile: work with cygwin on different windows versions.
.gitignore sanity.
Makefile: On Windows, executable files must end with .exe.
client.py: Windows files don't support ':', so rename cachedir.
index.py: os.rename() fails on Windows if dstfile already exists.
Don't try to rename tmpfiles into existing open files.
helpers.py: Cygwin doesn't support `hostname -f`, use `hostname`.
cmd-index.py: Retry os.open without O_LARGEFILE if not supported.
Makefile: Build on Windows under Cygwin.