Backport fvwm3 xthreadlock() patch to fvwm2

I’ve been getting regular fvwm2 crashes on various machines when, for example, exiting emacs and almost every time Zoom (meeting) quits. This is fixed by applying the fvwm3 xthreadlock() patch to fvwm2. This problem causes loss of data so is important.
See: debian bug 1034054: backport fvwm3 xthreadlock() by ThomasAdam · Pull Request #102 · fvwmorg/fvwm · GitHub
This really needs to urgently go in fvwm2 for a new fvwm2 release and the distros need informing. Remember Fedora uses fvwm2 and it will be regularly crashing for everyone and many users will not know why and give up as the cause is a little hard to find.

The patch has already been backported, it is in a PR. fvwm version 2 is no longer supported, but the patch is there to point distros who wish to still ship fvwm version 2 at. They should be able to apply it like debian has.