cookie: Add support for cookie prefixes

The draft-ietf-httpbis-rfc6265bis-02 draft, specify a set of prefixes
and how they should affect cookie initialization, which has been
adopted by the major browsers. This adds support for the two prefixes
defined, __Host- and __Secure, and updates the testcase with the
supplied examples from the draft.

Closes #3554
Reviewed-by: Daniel Stenberg <daniel@haxx.se>