(Fencode_time): Use mktime to do the real work;

(Fencode_time): Use mktime to do the real work;
this fixes bugs involving out-of-range dates and leap seconds,
and allows date arithmetic via out-of-range values for arguments.
Allow the ZONE parameter to be a TZ-style string.
Doc string fix: `1900' -> `this century'.
(set_time_zone_rule): New function.
(Fset_time_zone_rule): Use it.
(environ, make_time): Add extern declarations.
(days_per_month): Remove.