Configure error output for Lisp applications

Hello, Lisp developers.

initiated by some forum feedback, I have added some functionality to control the Lisp
engine's default error handler by Lisp code directly, regarding the mode and amount
of error output.

(vl-bt-on)    : enables "error callstack backtrace"
(vl-bt-off)   : disables "error callstack backtrace"
(vl-bt <n>) : outputs the "error callstack backtrace" to commandline, regardless of
                      enabled / disabled backtrace state; uses <n> error levels;
                      intended to be used from inside error handler, usually

This effects the behaviour of default *error* error handler, whether to show the
error callstack backtrace or not.

Additionally, there is now a "magic" Lisp variable which controls whether or not to
display the extra error location message :

(setq bcad$disable-extended-error t)   : disables output of error location message
(setq bcad$disable-extended-error nil) : enables output of error location message

Internal (startup) defaults :
- error callstack backtrace is on
- (setq bcad$disable-extended-error nil)

Using these 2 features, the error output can be reduced to minimum style (like AutoCAD)
or to maximum style incl. error callstack and error location message.

I hope this is a good solution ?
Will be available in an upcoming Bricscad release.