restore: catch task on the exit from sigreturn (v4)

A task is stopped here for unmaping restorer blob and restoring a state.

The method is the same as for parasite. CRIU attaches to processes via
ptrace and start to trace all syscalls.

v2: don't use a software breakpoint
v3: stop all thread on the exit from sigreturn
v4: attach to each thread
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>