Change the return value to unsigned, to match with the expected data type
where it is used.

Handle very large numbers consistently. Currently it was converting from
unsigned long to int, which would throw away the most significant
bits. Now overly large integers will be capped at UINT_MAX.

Implement the "allow and ignore decimal point" behaviour that the Age
header parsing incorporated in rfc2616_time(). This way we will allow a
decimal points also in max-age and stale-while-revalidate parsing of
Cache-Control directives.