* zdiff.in: Fix typo that broke most usages. Problem reported by

* zdiff.in: Fix typo that broke most usages.  Problem reported by
Jari Aalto in <http://bugs.debian.org/404114>.  While we're at it,
fix a bunch of other problems.  Handle "-" better.  Send
diagnostics to stderr, not stdout.  Use expr rather than echo |
sed, to handle special characters better.  Report a diagnostic in
the 1-arg case, if the argument doesn't end in .gz or the like,
rather than having incomprehensible behavior.  Do not require that
the inputs be regular files.  Avoid creating a temporary entirely,
if /dev/fd works.  If not, then resist denial-of-service attacks
better, by using mktemp.
* Makefile.am (gzip.doc.gz): New rule.
(check-local): Depend on it, and test zdiff for Debian bug 404114.