Assorted cleanups to Luke's cygwin fixes.

There were a few things that weren't quite done how I would have done them,
so I changed the implementation.  Should still work in cygwin, though.

The only actual functional changes are:
 - index.Reader.close() now actually sets m=None rather than just closing it
 - removed the "if rename fails, then unlink first" logic, which is
   seemingly not needed after all.
 - rather than special-casing cygwin to use "hostname" instead of "hostname
   -f", it turns out python has a socket.getfqdn() that does what we want.