Revert unintended change to ABI for extern functions.

* localtime.c (offtime, timeoff, gtime): Revert change to ABI, by
going back to 'long' instead of 'int_fast32_t' for types accepted
and returned by extern functions.  Reported by Christos Zoulas in
<http://mm.icann.org/pipermail/tz/2013-July/019488.html>.