Consolidate several near-identical uses of mktime() into a single

Consolidate several near-identical uses of mktime() into a single
routine DetermineLocalTimeZone().  In that routine, be more wary of
broken mktime() implementations than the original code was: don't allow
mktime to change the already-set y/m/d/h/m/s information, and don't
use tm_gmtoff if mktime failed.  Possibly this will resolve some of
the complaints we've been hearing from users of Middle Eastern timezones
on RedHat.