(TIME_SPEC_DATE): No longer needs to be nonzero, so

(TIME_SPEC_DATE): No longer needs to be nonzero, so
remove the "=1".
(TIME_SOEC_HOURS, TIME_SPEC_MINUTES): Must be at end now, so put
them there.
(time_spec_string, time_spec): Hours and minutes must be at
start now, so put them there.
(rfc_2822_format): Now a string constant, not a boolean.  All uses
changed.
(iso_8601_format, rfc_format): Remove.
(RFC_3339_OPTION): New constant.
(long_options): Add --rfc-3339.
(usage): Add --rfc-3339.  Don't mention --iso-8601.
Mention %:z, %::z, %:::z.
(main): Simplify calculation of 'format'; it was getting too hairy
to follow.  Add --rfc-3339.
(show_date): Assume format arg is not NULL, which is the case
now.  The default code is moved to 'main'.  This simplifies things
and allows the default to be calculated just once.