bug#12911: 24.3.50; let users decide where (& perhaps whether)`emacs_backtrace.txt' filesare written

[Drew Adams]

> > All my other programs store program-generated files under 
> > AppData. None writes indiscriminately to the current directory
> > in the event of a crash.
> 
> Do you have many Windows programs that do generate a backtrace file in
> the event of failure? And do they all write to %APPDATA%?
> 
> If the answer to both questions is "yes", are these Cygwin programs?

Why not add "and whose name is `Emacs'" while you're at it?

Seriously, this is not only about applications that generate a backtrace file.
It's about the etiquette that applications generally respect on Windows, in
order to respect the user and user data.

Why narrow it to applications that write backtrace files?  Is there something
particular about that case which should exclude it from respecting of the normal
etiquette?

Sure, if a program absolutely CANNOT respect the expected behavior because of
some hard constraint, then maybe that's a reason to make it an exception.  So
far, we haven't seen such a reason, AFAICT.  It might not be super simple for
Emacs to DTRT here, but that's not the same thing as saying that it CANNOT do
so.

And of course we have several decades of Emacs use without this new feature, in
which Emacs has not found it necessary to go beyond the pale.