crtools: Merge pstree collecting into showing

1. There's no need in collecting children list in shower
2. There's no need in reading file twice -- we can collect and
   show everything in one go.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>