[settings] Unregister the children when unregistering the parent

[settings] Unregister the children when unregistering the parent

The DHCP settings registered as a child of the netdevice settings are
not unregistered anywhere.  This prevents the netdevice from being
freed on shutdown.

Fix by automatically unregistering any child settings when the parent
settings are unregistered.

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>