Added a permissions check, to make sure that the user has the right to open the image file. Previously, if the user did not, the frisbeed process would not start properly, but nothing would complain.
Added a permissions check, to make sure that the user has the right to open
the image file. Previously, if the user did not, the frisbeed process would
not start properly, but nothing would complain.

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