ZFS_COLOR Use ANSI color in zfs diff and zfs list output.

[T-Daemon]

Stumbled by accident over the env variable browsing in zfs(8). Some of you might find it useful as I do.

https://github.com/openzfs/zfs/pull/14350 .

If the ZFS_COLOR env variable is set, then output zfs list is colored as follows:

• Header row is bold
• The AVAIL column is colored based on the percentage still available

...............yellow if < 20% available
...............red if < 10% available