fixups for systemd-osc-context (#43091)

I noticed that the use of sed in the `80-systemd-osc-context.sh` was
unnecessary, and upon writing a replacement in pure bash and testing it,
actually buggy. While in there, I also spotted some other minor things
that might be worth doing.

Getting rid of fork+exec for doing sed is probably the biggest win, but
there are still quite a few subshell invocations (that only do fork, not
exec). I haven't measured the overhead of those, but it seems that we
could get rid of those as well if we want.

Another thing I wonder about is whether we should emit an end= marker on
shell exit. There's no guaranteed way of doing it. Just as everything
else, we could get killed before getting a chance to do it. But also,
unlike PROMPT_COMMANDS, there's no "at_exit" array to hook into. Still,
we could ask if there is a 'trap exit' hook installed, and if not,
install one ourselves; if the user's bashrc subsequently overrides that,
so be it (though .bash_logout would be more appropriate).