dump: Checkpoint per-thread blocked signals

I've moved dump_thread helper a bit lower in file
since I've to call for find_thread_state helper.
After all this groups all thread related functions
in one slab.

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