add callback to allow additional server verification certs

add callback to allow additional server verification certs

This adds a LWS_CALLBACK_OPENSSL_LOAD_EXTRA_SERVER_VERIFY_CERTS callback
which offers a chance for the server context to be loaded with additional
certtificates allowing it to verify incoming client certs.  The callback
always comes to protocol[0].

It also introduces the context option LWS_SERVER_OPTION_REQUIRE_VALID_OPENSSL_CLIENT_CERT
which will enforce client cert checking on any ssl connection.

Signed-off-by: Andy Green <andy@warmcat.com>