Colorize zpool iostat output

Use a bold header and colorize the space suffixes in iostat
by order of magnitude like this:

• K is green
• M is yellow
• G is red
• T is lightblue
• P is magenta
• E is cyan
• 0 space is colored gray

Reviewed-by: WHR <msl0000023508@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ethan Coe-Renner <coerenner1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14621
Closes #14459