This is not really packaged for re-use. But I thought the code might be
interesting / helpful for others looking to process long gz/zstd streams in
C with forward seeking, rewind, and ability to ignore the checksum at the
end of the stream (gzip only).

Streams may be arbitrarily nested (i.e., gzip of zstd of gzip) but detection
is not (yet) automatic.  Automated detection can be performed simply by
repeatedly attempting zopenfile() and zstdopenfile().

License? See LICENSE.