fbdepth: enforce line-buffering on stdout

By default, it's block-buffered if stdout is non-interactive,
which affects ordering vs. stderr when using shell I/O redirections.

This made reading DevCap logs super-confusing.

utils/fbdepth.c

@@ -316,6 +316,10 @@ int
 	bool canonical_rota = false;
 #endif
 
+	// NOTE: Enforce line-buffering, to make I/O redirections less confusing (e.g., in DevCap logs),
+	//       as we often mix stdout with stderr, and unlike stdout, stderr is always unbuffered (c.f., setvbuf(3)).
+	setlinebuf(stdout);
+
 	while ((opt = getopt_long(argc, argv, "d:hvqgGr:R:oOcCH:", opts, &opt_index)) != -1) {
 		switch (opt) {
 			case 'd':