ptyfwd: optionally prefix window title with colored dot

in uid0/systemd-run/nspawn we already set a window title with a colorful
unicode dot indicating the changed privileges/execution context. This typically
gets overriden by the shell inside the environment however.

Let's tweak this a bit: when we see the window title OSC ANSI sequence
passing through, let's patch in the unicode dot as a prefix to the
title.

This is super pretty, since it makes sure root sessions via 0ad are
really easily recognizable as such, because the window title carries an
🔴 red dot as prefix then.

src/nspawn/nspawn.c

@@ -4443,6 +4443,9 @@ static void set_window_title(PTYForward *f) {
                 (void) pty_forward_set_titlef(f, "%sContainer %s on %s", strempty(dot), arg_machine, hn);
         else
                 (void) pty_forward_set_titlef(f, "%sContainer %s", strempty(dot), arg_machine);
+
+        if (dot)
+                (void) pty_forward_set_title_prefix(f, dot);
 }
 
 static int merge_settings(Settings *settings, const char *path) {