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

[Stefan Monnier]

> Accessing environment variables is another problematic place.  We are
> crashing, so the heap or the whole arena can be trashed.  Who can be
> sure the environment variables will not point to garbled places?

Just to put things in perspective: this backtrace "feature" was put in
to replace/supplement the previous assertion failure output (because
with asserts now being inside inlinable functions, the line&file info
we get is not the one we want any more).  So the environment is usually
still pretty sane, because assertions are usually caught fairly early.

Of course, there will be cases where the process is sufficiently botched
up that we can't build the file name ~/.emacs.d/backtrace.txt, while we
might still be able to just use "backtrace.txt" successfully, but
I don't think those borderline cases are sufficiently common to be
worry about them.


        Stefan