udt_relay: Add a forked copy of main to just relay messages

Do not run any of the header rewriting for messages. This will
simply relay SCCP friends to the MTP library. Some work on msc_conn.c
is needed to avoid calling most of it.
diff --git a/.gitignore b/.gitignore
index 84e69a9..1e469dc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,3 +25,4 @@
 mgcp_mgw
 patching_test
 mtp_parse_test
+udt_relay