gzip: make the GZIP env var obsolescent

gzip: make the GZIP env var obsolescent

* NEWS, gzip.1:
* doc/gzip.texi (Environment, Tapes): Document this.
* gzip.c (args): Remove static var; no longer needed now that
'main' frees it.  All uses removed.
(ENV_OPTION, shortopts): New constants.
(main): Warn about nontrivial uses of GZIP.  Reject dangerous uses.
* tests/gzip-env: New test case.
* tests/Makefile.am (TESTS): Add it.
* util.c (add_envopt): Create new vector instead of adding to old
one.  Only use changed.