fix various stuck-solo cases:

fix various stuck-solo cases:

* solo groups
* cancel-solo
* SIP <> AFL/PFL changes

The optimized plural-form route_solo_changed() relied on the false
premise that solo-groups and port-connections are disjoint sets.

-=-

e.g. "cancel all solo" calls set_solo(get_routes(), false);
Since All routes are affected, the "non_solo_change" set is empty, and
no changes were propagated up/downstream.

Routes that indirectly change state as group-members, wrongly end up in
the "non_solo_change" list instead of the "solo_change" list.

If a route feeds another in the same group, no changes were propagated.