xids: Save pgid and sid on pstree_Item and pstree_entry

I store them on _entry since sids can only be inherited or
set to current's pid. Thus the best we can do it restore sids
at fork time, thus save them in the image we use to fork.

Maybe when we submit patches that will give us ability to set
arbitrary pgid and sid we'll change this, but this is in the
future.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>