gzip: add "--keep" option to retain (don't delete) input files

gzip: add "--keep" option to retain (don't delete) input files

gzip now accepts the --keep (-k) option, for consistency with tools
like xz, lzip and bzip2.  With this option, gzip no longer removes
named input files when compressing and decompressing.
* doc/gzip.texi: Document it.
* gzip.1: Likewise.
* gunzip.in: Likewise.
* NEWS: Likewise.
* gzip.c: Add support for "--keep".