ns2: ensure link states are logged
when a ethernet device go down, ensure this is logged by osmocom as well a link recovery.
So far we only perform link monitoring within gprs_ns2_fr.c for the frame relay devices. I think there it also makes sense as those are basically "single use devices" where the entire FR/... stack is implemented by us.
For ethernet devices, this is somewhat more questionable. Ethernet, IP and TCP/UDP are implemented in the kernel. We don't even know whcih ethernet devices exist, what kind of routes there are, etc.
Shouldn't it be sufficient if those kind of events are logged in syslog/journald?
Do other "IP server prograns" customaril log the link state of ethernet interfaces?