zfsboot patch for /usr

[Roger Marquis]

> You don't mkdir it, you create it as a ZFS dataset, and mark it with the
> 'canmount=no' property, so it only exists to be a parent, not as an
> actual dataset. This is the default in zfboot currently.

Thanks to everyone for pointing this out.  I'll forget about mkdir then,
ignore the output of 'zfs list' and get comfortable doing things the zfs way.

Still have to tweak scripts/zfsboot to create a /var/spool subvol, a /home
subvol in place of /usr/home and specify atime=none in the default dataset. 
At least the latter works as expected.

Roger