
Fix up the /etc/passwd file creation process to work correctly when
the slice is rebooted (i.e., when rc.inplab gets re-run, don't append
all the existing service slice user entries - exponential blowup!).

This is what caused the bgmon sliver on the German node to go haywire with
memory - the sliver was rebooted many times.

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