Document all sorts of C debugging tips

Fulfills a pledge I made on on julia-dev. This moves some info I wrote from the FAQ to the "developer docs", and blatantly steals from sources posted by @ihnorton and @staticfloat. Finally, there's a small number of whitespace changes.

It might be because of the new REPL.
On Nov 13, 2014 12:19 PM, "Tim Holy" <notifications@github.com> wrote:

> In doc/devdocs/interactivegdb.rst:
>
> > @@ -0,0 +1,68 @@
> > +Interactive debugging with ``gdb``
> > +----------------------------------
> > +
> > +First, you should build the debug version of julia with ``make
> > +debug``.  Below, lines starting with ``(gdb)`` mean things you should
> > +type at the gdb prompt.
> > +
> > +From the shell
> > +^^^^^^^^^^^^^^
> > +
> > +The main challenge is that Julia and gdb each need to have their own
> > +terminal, to allow you to interact with them both.  One approach is to
>
> What's funny is that I'd swear Ctrl-C did not interrupt to gdb when I
> first tried it oh-so-long ago. So that's why I figured this out.
>
> I'll change this (probably just delete the whole section).
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/JuliaLang/julia/pull/9000/files#r20319864>.
>