cookies: leave secure cookies alone

Only allow secure origins to be able to write cookies with the
'secure' flag set. This reduces the risk of non-secure origins
to influence the state of secure origins. This implements IETF
Internet-Draft draft-ietf-httpbis-cookie-alone-01 which updates
RFC6265.

Closes #2956
Reviewed-by: Daniel Stenberg <daniel@haxx.se>