Check if AFS is mounted before killing processes with open files

Check if AFS is mounted before killing processes with open files

* Skip killing processes with files open in AFS if AFS does not appear
  to be mounted according to /etc/mtab.  Otherwise, we may call lsof
  without a specific mount point and kill far more processes than we
  intend to.  (This code is disabled by default, so this problem would
  only be seen by people who enabled it.)