Another round on SF patch 618135: gzip.py and files > 2G

Another round on SF patch 618135: gzip.py and files > 2G
The last round boosted "the limit" from 2GB to 4GB.  This round gets
rid of the 4GB limit.  For files > 4GB, gzip stores just the last 32
bits of the file size, and now we play along with that too.  Tested
by hand (on a 6+GB file) on Win2K.

Boosting from 2GB to 4GB was arguably enough "a bugfix".  Going beyond
that smells more like "new feature" to me.