Add support to gunzip for zip files that contain a single member.

Add support to gunzip for zip files that contain a single member.
This matches the behavior of GNU gzip and is most useful with "gunzip
-c" to pipe the uncompressed output.  It will not decompress a file
with more than one member unless in cat mode, in which case only
the first file is displayed.

To decompress a .zip file without the -c option, "-S .zip" must be
specified. The file name stored in the .zip file is not used unless
the -N option is specified. This is consistent with GNU gzip).

Does not increase the size of gzip on the install media.
OK jmc@ for documentation.