MEDIUM: ssl: add the possibility to use a global DH parameters file

This patch adds the ssl-dh-param-file global setting. It sets the
default DH parameters that will be used during the SSL/TLS handshake when
ephemeral Diffie-Hellman (DHE) key exchange is used, for all "bind" lines
which do not explicitely define theirs.
diff --git a/include/proto/ssl_sock.h b/include/proto/ssl_sock.h
index fa5eef5..4db516e 100644
--- a/include/proto/ssl_sock.h
+++ b/include/proto/ssl_sock.h
@@ -64,6 +64,9 @@
 struct tls_keys_ref *tlskeys_ref_lookupid(int unique_id);
 void tlskeys_finalize_config(void);
 #endif
+#ifndef OPENSSL_NO_DH
+int ssl_sock_load_global_dh_param_from_file(const char *filename);
+#endif
 
 #endif /* _PROTO_SSL_SOCK_H */