Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.
When we receive a migration, if rebinding to the new listen address and
port fails for a reason which might be fixable, rather than killing the
server we retry once a second. Also in this patch: non-overlapping log
messages and a fix for the client going away halfway through a sendfile
loop.
If the sender disconnects its socket before sending the disconnect
message, the destination should restart the migration process. This
patch makes sure that happens.
