Use stderr for 'no pools/datasets available' error

The 'zfs list' and 'zpool list' commands output the message
'no datasets/pools available' to stdout. This should go to
stderr and only the available datasets/pools should go to
stdout. Returning nothing to stdout is expected behavior
when there is nothing to list.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #581