movemail: don't dump core if the current time is outlandish

movemail: don't dump core if the current time is outlandish

* movemail.c (popmail): Check for mbx_delimit_begin failure.
(mbx_delimit_begin): Fail if the current time is so outlandish
that localtime would fail or asctime would have undefined
behavior.  Use strftime to avoid asctime undefined behavior.