resolved: be more careful with weird links with low MTUs

resolved: be more careful with weird links with low MTUs

Apparently CAN links will show up in rtnetlink with very low MTUs. We
shouldn't consider them relevant if no IP is spoken over them, since
these MTUs are irrelevant for us then.

Hence, let's check if there's an address assigned to the link before
considering its MTU.

As additional safety net filter out MTUs smaller than the minimum DNS
packet size, too.

Finally, in case we don't find any suitable interface MTU, let's default
to 1500 as the generic Ethernet MTU.

Fixes: #19396