Fix zpool iostat -w header names

The zpool iostat latency histograms (-w) has column names
'sync_queue' and 'async_queue', which do not match the man page, nor
the equivalent columns in average latency. Change the column
names to be 'syncq_wait' and 'asyncq_wait' to be consistent.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8338