Clear the circular buffer when ownership changes (as for physical nodes during swapout/swapin).
Clear the circular buffer when ownership changes (as for
physical nodes during swapout/swapin).

Also work around small race condition when starting up;
the acl file is created fter daemonization, and there is a
small window where the file exists but is not complete.
Use tmp file and rename to avoid the race.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome