Fix regression caused by c14ca14

The 'zfs load-key' command was broken for 'keyformat=passphrase'.
Use the correct output vars when stdin is an interactive terminal.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: adam moss <c@yotes.com>
Closes #10264
Closes #10265