Add unit tests for tor_timegm signed overflow,
tor_timegm and parse_rfc1123_time validity checks,
and correct_tm year clamping.
Unit tests (visible) fixes in bug 13476.
Check all date/time values passed to tor_timegm
and parse_rfc1123_time for validity, taking leap
years into account.
Improves HTTP header validation.
Avoid unlikely signed integer overflow in tor_timegm
on systems with 32-bit time_t.
Fixes bug 13476.