o Minor bugfixes:
    - Set the correct day of year value when the system's localtime(_r)
      or gmtime(_r) functions fail to set struct tm. Not externally visible.
      Fixes bug 13476.
    - Avoid unlikely signed integer overflow in tor_timegm on systems with
      32-bit time_t.
      Fixes bug 13476.
  o Minor enhancements (validation):
    - Check all date/time values passed to tor_timegm and parse_rfc1123_time
      for validity, taking leap years into account.
      Improves HTTP header validation.
      Implemented with bug 13476.
    - Clamp year values returned by system localtime(_r) and gmtime(_r)
      to year 1 in correct_tm. This ensures tor can read any values it
      writes out.
      Fixes bug 13476.
  o Minor enhancements (testing):
    - Add unit tests for tor_timegm signed overflow, tor_timegm and
      parse_rfc1123_time validity checks, correct_tm year clamping.
      Unit tests (visible) fixes in bug 13476.