Security framework.

Security callback: selects which parameters are permitted including
sensible defaults based on bits of security.

The "parameters" which can be selected include: ciphersuites,
curves, key sizes, certificate signature algorithms, supported
signature algorithms, DH parameters, SSL/TLS version, session tickets
and compression.

In some cases prohibiting the use of a parameters will mean they are
not advertised to the peer: for example cipher suites and ECC curves.
In other cases it will abort the handshake: e.g DH parameters or the
peer key size.

Documentation to follow...

ssl/s2_srvr.c

@@ -1053,6 +1053,12 @@ static int request_certificate(SSL *s)
 
 	i=ssl_verify_cert_chain(s,sk);
 
+	if (i > 1)
+		{
+		SSLerr(SSL_F_REQUEST_CERTIFICATE, i);
+		goto msg_end;
+		}
+
 	if (i > 0)	/* we like the packet, now check the chksum */
 		{
 		EVP_MD_CTX ctx;