Add backtrace support to the debug console.

Squashed commit of the following:

commit b3afbbf15839d5ad9343d4540674510cbd6cd16d
Author: Kelvin Sherlock <ksherlock@gmail.com>
Date:   Wed Dec 24 17:44:20 2014 -0500

    improve the debugger help a little bit

commit 82e1e4e3e4d802defbf49c965500ffc72c7be1af
Author: Kelvin Sherlock <ksherlock@gmail.com>
Date:   Wed Dec 24 17:32:21 2014 -0500

    prevent filename tab completion in the debugger.

commit 8765e5f428562e5ab6f8d59ec0e0460a834c66b5
Author: Kelvin Sherlock <ksherlock@gmail.com>
Date:   Wed Dec 24 16:06:21 2014 -0500

    skip macsbug names when disassembling via ;list

commit 9b87cfb3851fedc6423629608f53076741116991
Author: Kelvin Sherlock <ksherlock@gmail.com>
Date:   Wed Dec 24 15:43:13 2014 -0500

    improved backtracing.

commit f8e364d7c568fe6728c8efd5587f7edeb49f5816
Author: Kelvin Sherlock <ksherlock@gmail.com>
Date:   Wed Dec 24 15:17:28 2014 -0500

    BackTrace support