Issue OSC ANSI sequence whenever we change "context" of a TTY, i.e. acquire privs...

Issue OSC ANSI sequence whenever we change "context" of a TTY, i.e. acquire privs, enter container or VM or similar (#35224)

This is mostly a strawman to get a discussion going regarding how to
communicate to terminal emulators such as ptyxis about run0 (and nspawn,
and vmspawn, and moe) and what it does.

It's hierarchical and I think still relatively simple.

/cc @chergert