shutdown/shutdown.sh: loop over shutdown hooks until all succeed

Up until now, `_check_shutdown()` returns true if at least one of the shutdown hooks succeeded. Change this to only return true if *all* succeeded. To prevent an infinite loop, introduce an upper bound of 40 iterations.

To see where this rubber meets the road, consider a dm-crypt volume contained in a ZVOL, with the ZFS pool itself being backed by another dm-crypt volume. For the uninitiated, ZFS is not just a filesystem but also an LVM which may contain virtual block devices called ZVOLs. But that is irrelevant, the only thing that matters is that we have dm-crypt layered on ZFS layered on dm-crypt. Yes, that's a real-world example.

Imagine we have two shutdown hooks, `30-dm-shutdown.sh` and `30-export-zfs.sh`. To properly untangle these layers, `30-dm-shutdown.sh` will be invoked first. It should fail since it was only able to close one of two dm-crypt volumes. Next, `30-export-zfs.sh` will be invoked. It will properly export the ZFS pool and exit successfully.

At this point, `30-dm-shutdown.sh` should be invoked a second time to close the remaining dm-crypt volume. Turns out it isn't because `_check_shutdown()` returns true as one of the shutdown hooks, namely `30-export-zfs.sh`, exited successfully. That is fixed by this patch.