If syncfiles fails, keep it retrying.
Also, slow down sync checking to avoid hammering the system.
Further, randomized delay to spread highly synchronized requestors.
Block attempts to do multiple concurrent syncfile runs.
It is likely that a client connects from fe80::, which
is explicitly omitted from ssh principals.
This time, have the client provide all currently set IP addresses
and the server will make a determination.
There remains the possibility it misconfigures a nic and tries to use that,
inducing failure. One strategy would be to filter the addresses and
only provide from the 'current' interface. Another is to just take
the hit as the node is likely going to suffer a lot from such a
misconfiguration anyway.
