Fix EOVERFLOW-related problems noted by Chistos Zoulas.

See: http://mm.icann.org/pipermail/tz/2014-October/021692.html
* asctime.c (asctime_r): Remove now-unnecessary EOVERFLOW ifdef.
* localtime.c (timesub): Set errno to EOVERFLOW on overflow.
POSIX requires this.
(ctime, ctime_r): Return NULL instead of having undefined behavior
when the time stamp is out of struct tm range.  NetBSD does this,
the standards allow it, and it's nicer for users.
* private.h (EOVERFLOW): Default to EINVAL on ancient hosts that lack it.
* NEWS: Document this.