ptyfwd: optionally prefix window title with colored dot

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.