Fixes <https://bugs.gnu.org/29335>.
* guix/zlib.scm (close-procedure): Remove.
(make-gzip-input-port): Do (dup (fileno port)) to get a file descriptor
for 'gzdopen'. Close PORT before returning. Use 'gzclose' as the
'close' procedure of the returned port.
(make-gzip-output-port): Likewise.
