There are apparently some applications that want to only turn on OSC52
if they can be sure the terminal supports it.
https://github.com/contour-terminal/vt-extensions/blob/master/clipboard-extension.md
Seems harmless enough, though IMO the correct query mechanism for
runtime controllable settings is XTGETTCAP, but, let's be a
good citizen and co-operate. The overhead is not too large and I
have more important windmills to tilt at.
Fixes#8788