RE: backtrace sets print-level to 3

[Drew Adams]

> I think I'll just fix Fbacktrace to 1) restore the previous value upon
> exit instead of setting it to Qnil, and 2) use 8 instead of 3.

The print-(level|length) of backtraces should be under user control, i.e., at
the Lisp level or, better, interactively (e.g. option).  It should not be
hard-coded.

If we are not using the global values of `print-level' and `print-length' or,
better, similar-acting variables (options?) that are specific to backtraces, why
not?  How can a user control this aspect of the bactrace output appearance?