Colorize zpool status output

If the ZFS_COLOR env variable is set, then use ANSI color
output in zpool status:

• Column headers are bold
• Degraded or offline pools/vdevs are yellow
• Non-zero error counters and faulted vdevs/pools are red
• The 'status:' and 'action:' sections are yellow if they're displaying a warning.

This also includes a new 'faketty' function in libtest.shlib that is
compatible with FreeBSD (code provided by @freqlabs).

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #9340