Do not basename every block device.

We may need to consider something like ZOL does if we have an
equivalent of Linux's /dev/disk/by-*, but until then we should
only delete '/dev' from displayed vdev paths starting with
'/dev/disk' or '/dev/rdisk'. This lets us see the full path
to zvol vdevs (and therefore, to which pool they belong) when
imported using their symlinks in '/var/run/zfs/zvol/dsk/*/'.

For example, if a user imports

    zpool import -d /var/run/zfs/zvol/dsk/tank/ tankinatank

we will now display the vdev as '/var/run/zfs/zvol/dsk/tankinatank/myvol'
in 'zpool status' rather than just 'myvol'.

Also, since OS X symlinks var -> private/var, there's no reason
to display the '/private'.