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

tests/zfs-tests/include/commands.cfg

@@ -77,6 +77,7 @@ export SYSTEM_FILES_COMMON='arp
     rm
     rmdir
     scp
+    script
     sed
     seq
     setfacl

tests/zfs-tests/include/libtest.shlib

@@ -3916,3 +3916,18 @@ function stat_size #<path>
 		;;
 	esac
 }
+
+# Run a command as if it was being run in a TTY.
+#
+# Usage:
+#
+#    faketty command
+#
+function faketty
+{
+    if is_freebsd; then
+        script -q /dev/null "$@"
+    else
+        script --return --quiet -c "$*" /dev/null
+    fi
+}