mgcp_e1: finish E1 support, add E1 support from libosmoabis

Currently only the endpoint handling for E1 exists, but there is no
actual code behind it that handles the E1 traffic.

Change-Id: I6b93809b5ac7d01af55888347dd787b0bc997ae1
Related: OS#2659
diff --git a/src/libosmo-mgcp/mgcp_conn.c b/src/libosmo-mgcp/mgcp_conn.c
index 6802b91..8c7918e 100644
--- a/src/libosmo-mgcp/mgcp_conn.c
+++ b/src/libosmo-mgcp/mgcp_conn.c
@@ -397,3 +397,13 @@
 
 	return NULL;
 }
+
+/*! get oldest connection in the list.
+ *  \param[in] endp associated endpoint */
+struct mgcp_conn *mgcp_conn_get_oldest(struct mgcp_endpoint *endp)
+{
+	if (llist_empty(&endp->conns))
+		return NULL;
+
+	return llist_last_entry(&endp->conns, struct mgcp_conn, entry);
+}