Shows process tree from image. The output is like

    PID   PGID    SID   COMM
      1      1      1   session00
      4      4      4       session00
      7      7      7           session00
      8      4      4           session00
     11     11     11           session00
     12      4      4               session00
     13     13     13           session00
     14     14     14               session00
     15      4      4                   session00
      6      4      4       session00
     10      9      9       session00

(the above is for session00 test).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Appreciated-by: avagin@openvz.org