zpool: flush output before sleeping

Several zpool commands (status, list, iostat) have modes that present
some information, sleep a while, present the current state, sleep, etc.
Some of those had ways to invoke them that when piped would appear to do
nothing for a while, because non-terminals are block-buffered, not
line-buffered, by default. Fix this by forcing a flush before sleeping.

In particular, all of these buffered:

• zpool status <pool> <interval>
• zpool iostat -y<m> <pool> <interval>
• zpool list <pool> <interval>

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15593