Rather than parsing the seconds in an ISO 8601 date/time using a pair of
floating point numbers (numerator and denominator), use two integers
instead. This avoids issues around floating point precision, and also
makes it easier to check for potential overflow from overlong inputs.

This last point means that the `isfinite()` check can be removed, as it
was covering the case where a NAN was generated, which isn’t now
possible using integer arithmetic.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>