stack: handle external-children just like other states This also fixes color labels: previously when external-children was combined with another state, --color=debug would show something like: [topic.stack.index topic.stack.index.current - external-children|s4] Now there is a separate label for each state.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t