ensure SIGCHLD isn't inherited as blocked

Use SIG_SETMASK instead of SIG_BLOCKMASK here in case the parent had SIGCHLD
blocked. In this case if one of the criu threads has a problem, since the
SIGCHLD is blocked, the restore simply hangs.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>