cookie: max-age fixes

1 - allow >31 bit max-age values

2 - don't overflow on extremely large max-age values when we add the
value to the current time

3 - make sure max-age takes precedence over expires as dictated by
RFC6265

Bug: http://curl.haxx.se/mail/lib-2014-01/0130.html
Reported-by: Chen Prog