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

man/man8/zpool.8

@@ -433,6 +433,8 @@ to dump core on exit for the purposes of running
 .It Sy ZFS_COLOR
 Use ANSI color in
 .Nm zpool status
+and
+.Nm zpool iostat
 output.
 .It Sy ZPOOL_IMPORT_PATH
 The search path for devices or files to use with the pool.