criu: Dump and restore pdeath_sig value

The implementation is pretty straightforward. When dumping per-thread
misc data with parasite, collect one, then write in thread_core_info.

On restore wait for creds restore and put the value back (some creds
changes drop it to zero).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
zdtm: Tech that pdeath_sig is preserved

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
criu: Restore tasks as siblings in swrk

Andrey validly pointed out, that restoring pdeath_sig is not
compatible with criu_restore_child() call -- after criu restore
children, it will exit and fire the pdeath_sig into restored
tree root, potentially killing it.

The fix for that could be -- when started in swrk more, criu can
restore tree not as children tasks, but as siblings, using the
CLONE_PARENT flag when fork()-ing the root task.

With this we should also take care about errors handing -- right
now criu catches the SIGCHILD from dying children tasks, and
since we plan to create them be children of the criu parent (the
library caller) we will not be able to catch them. To do so we
SEIZE the root task in advance thus causing all SIGCHLD-s go to
criu, not to its parent.

Having this done we no longer need the SUBREAPER trick in the
library call -- tasks get restored right as callers kids :)

Some thoughts for future -- using this trick we can finally make
"natural" restoration of shell jobs. I.e. -- make criu restore
some subtree right under bash, w/o leaving itself as intermediate
task and w/o re-parenting the subtree to init after restore.

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