[states] fix strip but a case still does not work (see test test-state-strip.t)

[states] make enabling state saftier * Fix the lower state heads while enabling a state. * Add a --clever opiton that do not fix the lower heads (as earlier) * Add test for enable/disable state

[states] make disabling state saftier It is not allowed to disable a state that is not empty because "Explicit is better than implicit". A hint is displayed to help the user.