resolved: demote the global unicast scope

resolved: demote the global unicast scope

This will greatly reduce the number of cases where the global unicast
scope overlaps with link scopes configured as default-route, making it
feasible to use the global DNS setting in conjunction with per-link dns
servers configured by the network.

This change is preferred over demoting links to default-route=no where
the user prefers to use the network provided DNS servers, and I expect
it is non-disruptive in that it should not degrade the efficacy of any
existing configuration.