In cal-dst, be consistent re default to UTC

In cal-dst, be consistent re default to UTC

* lisp/calendar/cal-dst.el (calendar-standard-time-zone-name)
(calendar-daylight-time-zone-name):
When using alphabetic time zone abbreviations, default to "UTC"
rather than to "EST" or "EDT", to be consistent with the behavior
when using numeric time zone abbreviations.
Also, in the numeric time zone use "-0000" rather than "+0000"
to show that the time zone is unknown; this is the RFC 5322
standard.