Fixed the signed/unsigned confusions when dealing with files >= 2GB.
4GB is still a hard limitation of the gzip file format, though.

Testing this was a bitch on Win98SE due to frequent system freezes.  It
didn't freeze while running gzip, it kept freezing while trying to *create*
a > 2GB test file!  This wasn't Python's doing.  I don't know of a
reasonable way to test this functionality in regrtest.py, so I'm not
checking in a test case (a test case would necessarily require creating
a 2GB+ file first, using gzip to zip it, using gzip to unzip it again,
and then compare before-and-after; so >4GB free space would be required,
and a loooong time; I did all this "by hand" once).

Bugfix candidate, I guess.