lib: Implement the criu_restore_child() call v2

It fully uses the swrk action of criu. The problems, that caller may
have is that the restored tasks die _before_ libcriu's call returns.

v2:
* rename _sub to _child
* unblock sigchild before execl-ing criu

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>