In 2.2, 'fport_close' no longer swallows EBADF and instead raises a
'system-error' for this. This commit adjusts for 2.2.
* guix/zlib.scm (close-procedure): Remove.
(make-gzip-input-port): Use 'port->fdes' instead of 'fileno'.
Use 'gzclose' instead of 'close-procedure'.
(make-gzip-output-port): Likewise.
* tests/zlib.scm ("compression/decompression pipe"): Don't check whether
PARENT is closed using 'port-closed?'. Instead, use 'seek' on the
underlying FD and check for EBADF.
* guix/zlib.scm (gzread!): Augment docstring to clarify when zero is
returned (based on reading zlib code).
(make-gzip-input-port)[read!]: Remove scary comment.
