loader: add support for chain and device BE's
Add support for chainload and device (as opposed to dataset) based boot environments in BE menu and beadm command.
When the entry type chain is activated, the loader will attempt chainload; the device entry does behave same as zfs BE - we set currdev variable and reload configuration from the new root.
In both cases it is user responsibility to use the correct device names.
Also beadm list will show the entry types.