resolved: rework dns_server_limited_domains(), replace by dns_scope_has_route_only_do...

resolved: rework dns_server_limited_domains(), replace by dns_scope_has_route_only_domains()

The function dns_server_limited_domains() was very strange as it
enumerate the domains associated with a DnsScope object to determine
whether any "route-only" domains, but did so as a function associated
with a DnsServer object.

Let's clear this up, and replace it by a function associated with a
DnsScope instead. This makes more sense philosphically and allows us to
reduce the loops through which we need to jump to determine whether a
scope is suitable for default routing a bit.