dns: auto-activate down ifaces only for desired link-local DNS and iif

Restore auto-activation of managed/disconnected interfaces when they
are explicitly referenced by desired IPv6 link-local DNS
(fe80::…%iface) or route-rule iif, without requiring
interfaces: [{name, state: up}].

Limit that behavior so inherited current DNS does not activate
interfaces, and mark_as_up_for_apply() skips already desired or
changed ifaces (e.g. orphan VLANs marked absent when a parent bond
is removed). Keep rejecting unmanaged/ignored ifaces and leave
search/options off arbitrary down ifaces.

Resolves: https://redhat.atlassian.net/browse/NMT-1893

Assisted-By: Claude Sonnet 5
Signed-off-by: Branislav Kadlec <bkadlec@redhat.com>