possible time zone file syntax extension

[Arthur David Olson]

There's work in progress on adding historical information to time zone files.
It might be useful to extend the syntax of "UNTIL" parts so that you could
write something such as

	# Zone	NAME	GMTOFF	RULES/SAVE	FORMAT	[UNTIL]
	Zone	This	0:00	-		TST	1916 May 21 2:00s
			0:00	Whatever	T%sT	1968 Feb 18 2:00s->That

where the "->That" on the final line means "after the UNTIL time,
use whatever happens in Zone 'That'."  This would both allow for more
concise representation of historic information and allow a "-c"
(or some other letter) flag to zic that would tell it to only compile
"current" zones--zones that did NOT end with a line containing "->".

If you have insights on whether this is the best thing to do, I'd love it if
you'd share them with folks on the time zone mailing list.

				--ado