Add gzip based on libz. This is faster for decompression and yields

Add gzip based on libz. This is faster for decompression and yields
better compression than GNU gzip (300 KB for the plan9 ISO image).

The optional support for decompression of compress and bzip2 is disabled,
the normal frontends should be used instead. They can be activated by
setting GZIP_UNCOMPRESS and GZIP_UNBZIP2 respectively.

The only missing features from GNU gzip are the omission of zip support
(it could only handle the first entry anyway) and unpack support.

This differs from the NetBSD version in that all compile time warnings
has been fixed.

Obtained-from: NetBSD