Update plamo template

- If "installpkg" command does not exist, lxc-plamo temporarily
  install the command with static linked tar command into the lxc
  cache directory.  The tar command does not refer to passwd/group
  files, which means that only a few files/directories are extracted
  with wrong user/group ownership.  To avoid this, the installpkg
  command now uses the standard tar command in the system.
- Change mode to 666 for $rootfs/dev/null to allow write access for
  all users.
- Small fix in usage message.

Signed-off-by: TAMUKI Shoichi <tamuki@linet.gr.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: KATOH Yasufumi <karma@jazz.email.ne.jp>