[m-dev.] for review: add `--debug' grade

[Zoltan Somogyi]

> Zoltan, can you please review this one?

> +To try this out, compile your program with @samp{--debug},
> +run @samp{mdb @var{program}}, and then type @samp{?}
> +at the @samp{mdb} prompt.

At the moment, the ? command lists all the breakpoints. The help command is h.
However, I just replaced the command parser in the debugger, and we can now
change command names more freely.

I also just added a new debugger command to print the stack trace, so execution
tracing now requires stack tracing.

Apart from that, the change is fine.

Zoltan.