Add basic backlight percentage support

At the moment linux only, but will add support for OpenBSD as well.

components/backlight.c

@@ -0,0 +1,32 @@
+/* See LICENSE file for copyright and license details. */
+#include "../util.h"
+
+#if defined(__linux__)
+	#include <limits.h>
+
+	#define BRIGHTNESS_MAX "/sys/class/backlight/%s/max_brightness"
+	#define BRIGHTNESS_CUR "/sys/class/backlight/%s/brightness"
+
+	const char *
+	backlight_perc(const char *card)
+	{
+		char path[PATH_MAX];
+		int max, cur;
+
+		if (esnprintf(path, sizeof (path), BRIGHTNESS_MAX, card) < 0 ||
+			pscanf(path, "%d", &max) != 1) {
+			return NULL;
+		}
+
+		if (esnprintf(path, sizeof (path), BRIGHTNESS_CUR, card) < 0 ||
+			pscanf(path, "%d", &cur) != 1) {
+			return NULL;
+		}
+
+		if (max == 0) {
+			return NULL;
+		}
+
+		return bprintf("%d", cur * 100 / max);
+	}
+#endif