Implement the Opaque PRF Input TLS extension

(draft-rescorla-tls-opaque-prf-input-00.txt), and do some cleanups and
bugfixes on the way.  In particular, this fixes the buffer bounds
checks in ssl_add_clienthello_tlsext() and in ssl_add_serverhello_tlsext().

Note that the opaque PRF Input TLS extension is not compiled by default;
see CHANGES.

ssl/ssl_lib.c

@@ -542,6 +542,7 @@ void SSL_free(SSL *s)
 	if (s->tlsext_ecpointformatlist) OPENSSL_free(s->tlsext_ecpointformatlist);
 	if (s->tlsext_ellipticcurvelist) OPENSSL_free(s->tlsext_ellipticcurvelist);
 #endif /* OPENSSL_NO_EC */
+	if (s->tlsext_opaque_prf_input) OPENSSL_free(s->tlsext_opaque_prf_input);
 #endif
 
 	if (s->client_CA != NULL)