The loop is run after the main interface DNS was set, thus the error
would make noise to users. This commit removes the noise, by making
currentStaticDNS returns an additional error, so it's up to the caller
to decive whether to emit the error or not.
Further, the physical interface loop will now only log when the callback
function runs successfully. Emitting the callback error can be done in
the future, until we can figure out how to detect physical interfaces in
Go portably.
The save/restore DNS functionality always perform its job, even though
the DNS is not static, aka set by DHCP. That may lead to confusion to
users. Since DHCP settings was changed to static settings, even though
the namesers set are the same.
To fix this, ctrld should save/restore only there's actual static DNS
set. For DHCP, thing should work as-is like we are doing.
On some routers, change to network may trigger re-rendering
/etc/resolv.conf file, causing requests from router itself stop using
ctrld.
Fixing this by watching changes to /etc/resolv.conf, then revert them.
