Add colored output to zfs list

- Bold header row
- Color AVAIL column based on percentage of volume available
	- < 20%: Yellow
	- < 10%: Red

Signed-off-by: Ethan Coe-Renner <coerenner1@llnl.gov>

cmd/zfs/zfs_main.c

@@ -3440,6 +3440,8 @@ print_header(list_cbdata_t *cb)
 	boolean_t first = B_TRUE;
 	boolean_t right_justify;
 
+	color_start(ANSI_BOLD);
+
 	for (; pl != NULL; pl = pl->pl_next) {
 		if (!first) {
 			(void) printf("  ");
@@ -3466,9 +3468,31 @@ print_header(list_cbdata_t *cb)
 			(void) printf("%-*s", (int)pl->pl_width, header);
 	}
 
+	color_end();
+
 	(void) printf("\n");
 }
 
+/*
+ * Decides on the color that the avail value should be printed in.
+ * > 80% used = yellow
+ * > 90% used = red
+ */
+static const char *
+zfs_list_avail_color(zfs_handle_t *zhp)
+{
+	int used = zfs_prop_get_int(zhp, ZFS_PROP_USED);
+	int available = zfs_prop_get_int(zhp, ZFS_PROP_AVAILABLE);
+	int percentage = (int)((double)available / (available + used) * 100);
+
+	if (percentage > 20)
+		return (NULL);
+	else if (percentage > 10)
+		return (ANSI_YELLOW);
+	else
+		return (ANSI_RED);
+}
+
 /*
  * Given a dataset and a list of fields, print out all the properties according
  * to the described layout.
@@ -3531,6 +3555,9 @@ print_dataset(zfs_handle_t *zhp, list_cbdata_t *cb)
 			right_justify = B_FALSE;
 		}
 
+		if (pl->pl_prop == ZFS_PROP_AVAILABLE)
+			color_start(zfs_list_avail_color(zhp));
+
 		/*
 		 * If this is being called in scripted mode, or if this is the
 		 * last column and it is left-justified, don't include a width
@@ -3542,6 +3569,9 @@ print_dataset(zfs_handle_t *zhp, list_cbdata_t *cb)
 			(void) printf("%*s", (int)pl->pl_width, propstr);
 		else
 			(void) printf("%-*s", (int)pl->pl_width, propstr);
+
+		if (pl->pl_prop == ZFS_PROP_AVAILABLE)
+			color_end();
 	}
 
 	(void) putchar('\n');

lib/libzfs/libzfs_util.c

@@ -2010,7 +2010,7 @@ use_color(void)
 void
 color_start(const char *color)
 {
-	if (use_color()) {
+	if (color && use_color()) {
 		fputs(color, stdout);
 		fflush(stdout);
 	}

man/man8/zfs.8

@@ -740,6 +740,8 @@ command will be undone if the share is ever unshared (like via a reboot).
 .It Sy ZFS_COLOR
 Use ANSI color in
 .Nm zfs Cm diff
+and
+.Nm zfs Cm list
 output.
 .It Sy ZFS_MOUNT_HELPER
 Cause