gzip: support the --rsyncable option

gzip: support the --rsyncable option

* deflate.c: Include verify.h.
(RSYNC_WIN, RSYNC_SUM_MATCH): Define.
(rsync_sum, rsync_chunk_end): Declare file-scoped globals.
(lm_init): Initialize globals.
(fill_window): Update rsync_chunk_end.
(rsync_roll): New function.
(RSYNC_ROLL): New macro.
(FLUSH_BLOCK): Update for new "pad" parameter.
(deflate_fast): Use RSYNC_ROLL and flush/pad.
(deflate): Likewise.
* trees.c (flush_block): Add "pad" parameter.
* gzip.c (rsync): New global.
(RSYNCABLE_OPTION, longopts, help): Add the option.
(main): Set the new global.
* gzip.h (rsync): Declare new global.
(flush_block): Update prototype.
* doc/gzip.texi: Document it.
* gzip.1: Likewise.
* bootstrap.conf: Use verify module.
* NEWS (New feature): Mention it.
* Makefile.am (check-local): Add tests and use AM_V__* command-
hiding opions. Reported against Debian here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=118118