* cal-dst.el: New file.
(calendar-/, calendar-%, calendar-absolute-from-time,
calendar-time-from-absolute, calendar-next-time-zone-transition,
calendar-time-zone-daylight-rules): New functions.
(calendar-current-time-zone): Moved from calendar.el and rewritten.
(calendar-current-time-zone-cache): New variable.
(calendar-current-time-zone, calendar-time-zone,
calendar-daylight-time-offset, calendar-standard-time-zone-name,
calendar-daylight-time-zone-name,
calendar-daylight-savings-starts, calendar-daylight-savings-ends,
calendar-daylight-savings-switchover-time): Moved from calendar.el.