libbe(3): Handle non-ZFS rootfs better

If rootfs isn't ZFS, current version will emit an error claiming so and fail
to initialize libbe. As a consumer, bectl -r (undocumented) can be specified
to operate on a BE independently of whether on a UFS or ZFS root.

Unbreak this for the UFS case by only erroring out the init if we can't
determine a ZFS dataset for rootfs and no BE root was specified. Consumers
of libbe should take care to ensure that rootfs is non-empty if they're
trying to use it, because this could certainly be the case.

Some check is needed before zfs_path_to_zhandle because it will
unconditionally emit to stderr if the path isn't a ZFS filesystem, which is
unhelpful for our purposes.

This should also unbreak the bectl(8) tests on a UFS root, as is the case in
Jenkins' -test runs.

MFC after: 3 days