This allows testing Y2038 with system time set to after that, so that
actual Y2038 issues can be exposed, and not masked by expiry errors.
Fixes#11576Closes#11610
Add test 676 to verify that setting CURLOPT_COOKIEFILE to NULL again clears
the cookiejar from memory.
Reported-by: Stefan Karpinski
Fixes#6889Closes#6891