date: support parsing of ISO-8601-with-"T" dates

date: support parsing of ISO-8601-with-"T" dates

Thanks to an improvement in gnulib's parse-datetime module,
commands like this now succeed (output manually indented):
    $ ./date -u -d 2004-02-29T16:21:42.33+07:00 +%FT%T.%N%z
		   2004-02-29T09:21:42.330000000+0000
* tests/misc/date: Add a test to exercise the new-in-gnulib
parsing of ISO8601-with-"T" dates.
* NEWS (New features): Mention it.
* gnulib: Update, to pull in this parse-datetime improvement.