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/src/libmgcp/mgcp_vty.c b/openbsc/src/libmgcp/mgcp_vty.c
index 42d99e7..8633668 100644
--- a/openbsc/src/libmgcp/mgcp_vty.c
+++ b/openbsc/src/libmgcp/mgcp_vty.c
@@ -1047,7 +1047,7 @@
 	}
 
 	endp = &trunk->endpoints[endp_no];
-	mgcp_free_endp(endp);
+	mgcp_release_endp(endp);
 	return CMD_SUCCESS;
 }