Add documentation for -T and interval to "zpool list"

zpool list has the same options for repeating as zpool iostat
has, but that is not documented. This patch adds the documentation.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Add missing -v to usage help for zpool list.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fix 'zpool list -H' error code

Due to an uninitialized variable it was possible for the command
'zpool list -H' to return a non-zero error when there are no pools.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1605