sd-bus: Preserve interfaces addition order

When adding a new interface to the object add it at the end of the list.
This way, when iterating over the list, e.g., during handling introspect
call, the order of returned interfaces will mach the order in which they
were added.

src/libsystemd/sd-bus/bus-objects.c

@@ -1973,6 +1973,9 @@ static int add_object_vtable_internal(
                 }
         }
 
+        if (!existing)
+                existing = LIST_FIND_TAIL(vtables, n->vtables);
+
         s->node_vtable.node = n;
         LIST_INSERT_AFTER(vtables, n->vtables, existing, &s->node_vtable);
         bus->nodes_modified = true;

test/units/TEST-23-UNIT-FILE.oneshot-restart.sh

@@ -90,9 +90,9 @@ read -r x <"$FIFO_FILE"
 assert_eq "$x" "finished"
 
 cmp -b <(systemctl show "$UNIT_NAME" -p Result -p NRestarts -p SubState) <<EOF
+SubState=dead
 Result=success
 NRestarts=1
-SubState=dead
 EOF
 
 systemctl disable "$UNIT_NAME"