Simplify restart logic and fix races

[Oren Laadan]

This series simplifies adn tightens the restart logic and solves a
couple of races that annoyed Serge recently:

*  c/r: simplify logic of tracking restarting tasks
*  c/r: let entire thread group in sys_restart before restoring a thread
*  c/r: introduce walk_task_subtree() to iterate through descendants
*  c/r: fix restart failure due to a race with ghost/dead tasks
*  c/r: defer restore of blocked signals mask during restart

Applies on ckpt-v18, works well on my tests.

Oren.