date: add '--debug' option

date: add '--debug' option

Usage example:
    date --debug -d 'now + 3 days'

Print parsing and debugging information to stderr when using date's
-d/--date option.
See: http://lists.gnu.org/archive/html/coreutils/2016-01/msg00076.html

* src/date.c (main): Add '--debug' option, enable debugging
in gnulib's parse-datetime.y module.
* tests/misc/date.pl: Repeat tests with '--debug' enable, ensure
no regression.
* tests/misc/date-debug.sh: Test output of '--debug' option.
* tests/local.mk: Add above test.
* NEWS: Mention new option.
* doc/coreutils.texi: Likewise.