anymore, use the low-level inflate()/deflate() directly. this
allows support file-type detection on stdin; now can decompress
.Z and .bz2 files fed to stdin (fixes PR#25192) additionally it
makes -v work with stdin or stdout (fixes PR#25215.)
(with these changes, we no longer need gzopenfull(3) in libz.)
thanks to martin@ for the code gzip.c:gz_uncompress() is based on.