S2S-TLS/OpenSSL: Postpone verification of TLS session right before server handshake

S2S-TLS/OpenSSL: Postpone verification of TLS session right before server handshake

The verify callback in OpenSSL is called pretty early, and at that time
it is not possible yet to check which connection it belongs to, and some
connections may have relaxed requirements.

So always return success in the Verify_openssl() callback, and postpone
validation of the TLS session until starting the server handshake in
cb_connserver_login_ssl(), when we know which server this connection
belongs to and which options (like "SSLVerify") are in effect.

The code doing this was already present in cb_connserver_login_ssl(),
but this patch adds a more prominent comment to the function.