*tp > LONG_MAX is never true, so replace that check with a test
for strtol() setting errno == ERANGE (oddly, some ports' build
swallowed this without warning).
There's no guarantee that a time_t stores the same number of bits
as a long, so check for an overflow there as well, and pass any
error up.
Discussed with christos, martin and mrg.
1bb5735b5e