ctrools: prepare to dump pid namespace

Add struct pid and use it everywhere. This struct contains
two fields: pid and real_pid.

real_pid is a pid outside of the target pid namespace.
pid is in the target pid namespace

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>