mgcp: Change API to remove memory management from the name

Jacob pointed out that "free_endp" refers to the memory of
the endpoint being freed. What we want is actually a way to
release an endpoint (and the resource it allocated) or in
the case of the testcase/testapp initialize the data structure
correctly. Introduce two names for that.
diff --git a/openbsc/tests/mgcp/mgcp_test.c b/openbsc/tests/mgcp/mgcp_test.c
index 5f35266..50c8b2d 100644
--- a/openbsc/tests/mgcp/mgcp_test.c
+++ b/openbsc/tests/mgcp/mgcp_test.c
@@ -828,12 +828,7 @@
 
 	endp.tcfg = &trunk;
 
-	/* This doesn't free endp but resets/frees all fields of the structure
-	 * and invokes mgcp_rtp_end_reset() for each mgcp_rtp_end. OTOH, it
-	 * expects valid pointer fields (either NULL or talloc'ed), so the
-	 * memset is still needed. It also requires that endp.tcfg and
-	 * trunk.endpoints are set up properly. */
-	mgcp_free_endp(&endp);
+	mgcp_initialize_endp(&endp);
 
 	rtp->payload_type = 98;