pstree: Move struct pid to pstree.h

I believe this make sense to keep this structure
in pstree.h where pstree related data lives.

Also I've added some comments on struct pid members.

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