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.

src/resolve/resolved-dns-scope.c

@@ -778,7 +778,8 @@ DnsScopeMatch dns_scope_good_domain(
                 if (!dns_scope_is_default_route(s))
                         return DNS_SCOPE_NO;
 
-                return DNS_SCOPE_MAYBE;
+                /* Prefer suitable per-link scopes where possible */
+                return s->link ? DNS_SCOPE_MAYBE : DNS_SCOPE_LAST_RESORT;
         }
 
         case DNS_PROTOCOL_MDNS: {