Fix localtime.c undefined behaviors and set errno.

Christos Zoulas reported a crash due to a tzsetlcl failure to
initialize data in some places, and requested that errno be set
when time functions fail; see:
http://mm.icann.org/pipermail/tz/2014-October/021754.html
While fixing this in a different way, I noticed and fixed another
instance of undefined behavior when read returns a too-small value.
* NEWS: Document this.
* localtime.c (union input_buffer): Rename from u_t.
(union input_buffer, union local_storage):
Move to top level so that two functions can use them.
(tzloadbody): New function, with most of the body of the old tzload.
Check for short reads that leave uninitialized buffers behind.
Define a new constant TZHEADSIZE for this, and use it to simplify
other code that already uses the concept.
(tzload): Use it.  This removes the need for gotos.  Return an errno
value; all callers changed.
(zoneinit): Return bool, not struct state *.  Assume SP is nonnull.
All callers changed.
(zoneinit, tzalloc): Set errno on failure.
(tzsetlcl): Don't crash if zoneinit fails.
* private.h (ENAMETOOLONG): Define if not already defined.