Include <stdbool.h>. Use bool where appropriate, instead of int.

Include <stdbool.h>.  Use bool where appropriate, instead of int.
(my_strftime): Do not mishandle years close to INT_MAX, by doing
the right thing even if adding 1900 would overflow.  Similarly
for tm_mon + 1 and tm_yday + 1.
Make %Y always equivalent to %C%y, and similarly for %G and %g.
(DO_NUMBER, DO_NUMBER_SPACEPAD): Set digits to d, not a conditional.
(DO_SIGNED_NUMBER): New macro.
(my_strftime) [HAVE_TZNAME]: Don't dump core if tp->tm_dst > 1.