zdump -v factor-of-2 speedup on 64-bit hosts (-500 -> 1800)

[Paul Eggert]

The command "zdump -v US/Pacific" took 36 CPU seconds on my
(admittedly aging) 440 MHz UltraSPARC-IIi machine.  There are no doubt
other speedups that could be done, but the following obvious one
shrank the CPU time to 20 CPU seconds.  Standard time wasn't
introduced before 1800 anywhere that I know of.

--- zdump.8	2006/02/20 15:08:17	2006.2
+++ zdump.8	2006/02/23 17:29:26	2006.2.0.1
@@ -41,7 +41,7 @@ otherwise.
 .BI "\-c " [loyear,]hiyear
 Cut off verbose output near the start of the given year(s).
 By default,
-the program cuts off verbose output near the starts of the years -500 and 2500.
+the program cuts off verbose output near the starts of the years 1800 and 2500.
 .SH LIMITATIONS
 The
 .B \-v
--- zdump.c	2006/02/21 21:07:30	2006.2.0.1
+++ zdump.c	2006/02/23 17:29:26	2006.2.0.2
@@ -18,7 +18,7 @@ static char	elsieid[] = "@(#)zdump.c	8.1
 #endif /* !defined isascii */
 
 #ifndef ZDUMP_LO_YEAR
-#define ZDUMP_LO_YEAR	(-500)
+#define ZDUMP_LO_YEAR	1800
 #endif /* !defined ZDUMP_LO_YEAR */
 
 #ifndef ZDUMP_HI_YEAR