bug#6758: 23.2; xterm.el: please provide an option to not discard input in terminal-init-xterm

[Stefan Monnier]

> Forget the time-dependent part of what I said, let's just send the
> query and handle the response whenever it happens to come in.  See
> the below patch.  This fixes everything -- it still detects both
> modifyOtherKeys and the background color, but doesn't require flushing
> input or rely on any timeouts at all, unlike the existing code.
> And there's nothing tricky.  What do you think?

Looks pretty good from here, thank you.
We may want to get rid of the "\e[>" (and "\e]11;") decode rules after
they've been used, just in case (or better yet: make them more
robust).

Any objection?


        Stefan