pstree: move state from pstree_item to pid

I'm going to collect all pids in rbtree, but threads doesn't have
pstree-entries, so we need a field to distinguish pstree_item-s from
threads.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>