Add a sanity limit to stack backtraces

[bnagy]

I don't think anyone needs to see 60k stack frames, and it takes forever. I capped it conservatively at 1000 by adding

            if i > 1000:
                break

to the end of lib/gdb_wrapper/x86.py in __init__ for Backtrace, which is horribly hacky but I don't the python.

[jfoote]

That makes sense. Will add a configurable default. Thanks for the report; I'll close this once a fix is in place.

[jfoote]

I added an option to the exploitable command (-b) that limits the number of frames included processed in the backtrace. 0 sets it back to unlimited, the default is 1000.