vtls: declare Curl_ssl structs for every SSL backend

vtls: declare Curl_ssl structs for every SSL backend

The idea of introducing the Curl_ssl struct was to unify how the SSL
backends are declared and called. To this end, we now provide an
instance of the Curl_ssl struct for each and every SSL backend.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>