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next few years until at most one rule remains; if none remain, then
just use the first candidate rule; it's wrong in general, but it's
right for at least one year.  This is a better heuristic in case the
underlying time zone implementation has bugs (which is all too
common).  If possible, don't convert back and forth between gregorian
and absolute; this speeds things up noticeably.  This uses the new
calendar-nth-named-absday function.

(calendar-current-time-zone): Some locales start DST at a different
time of day than they end; allow for this by yielding both times.  The
performance speedups in calendar.el are great enough that we now no
longer need the "Checking time zone data..." message.  If
current-time-zone yields nil, don't bother with
calendar-next-time-zone-transition.  Use clearer names for local vars.

(calendar-time-zone, calendar-daylight-time-offset,
calendar-{standard,daylight}-time-zone-name,
calendar-daylight-savings-{starts,ends}): Default to US Eastern rules
for information that is not available.

(calendar-daylight-savings-{starts,ends}-time): New vars, replacing
calendar-daylight-savings-switchover-time, to support locales that
start DST at a different time of day than they end.

(calendar-absolute-from-time):
Fix typo by interchanging floor and mod.