bug#39687: 26.3; Add customize-variable option for not locking keyboards

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  39687@debbugs.gnu.org
> Date: Thu, 01 Oct 2020 20:44:30 +0200
> 
> Logan Perkins <logan@lp-programming.com> writes:
> 
> > I *think* I have a patch that still fixes the current behavior, and
> > does not reintroduce those two bugs, I've included it below.
> > Basically, the fix for Bug#5095 should only be applied if we are in
> > the right context.  If we're not, the if block above puts a
> > Qswitch_frame at the head of the side queue and triggers
> > replay_entire_sequence, so we just skip the second check.  It'll get
> > run again and catch the interruption on the next pass, but in the
> > right context.
> 
> Eli, did you have a chance to take a look at the proposed patch here?

I didn't feel that I understand the issue well enough, and this area
is notorious for having all kinds of dark corners.