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

include/libzutil.h

@@ -146,6 +146,18 @@ extern int zpool_history_unpack(char *, uint64_t, uint64_t *, nvlist_t ***,
 struct zfs_cmd;
 int zfs_ioctl_fd(int fd, unsigned long request, struct zfs_cmd *zc);
 
+/*
+ * List of colors to use
+ */
+#define	ANSI_RED	"\033[0;31m"
+#define	ANSI_YELLOW	"\033[0;33m"
+#define	ANSI_RESET	"\033[0m"
+#define	ANSI_BOLD	"\033[1m"
+
+void color_start(char *color);
+void color_end(void);
+int printf_color(char *color, char *format, ...);
+
 #ifdef	__cplusplus
 }
 #endif