loader: export boot pool and vdev GUID
We do export zfs-bootfs, bootpath and diskdevid in boot environment to support zfs boot. To support boot pool discovery in case the bootpath and diskdevid are changed (are not reflecting the actual device tree), we need to export at least boot pool GUID. While on it, we also export boot disk vdev GUID. Once we can discover the boot pool configuration, we probably should not even need vdev GUID any more, because pool should should ensure the data integrity.
Export pool guid in zfs-bootpool
and vdev guid in zfs-bootvdev
While there, remove unused fbsd vfs.* variables.