GDB library + test + interactive support + plugin

Looks like we can review everything at once.

This PR includes what was in the first one,  the Avocado GDB library and its accompanying test, plus the interactive support (user level). 

I think that the `runtime` approach is cleaner, even though I agree there's some duplication here. *But*, if you think about `CURRENT_TEST`, then this approach gets a bit more valid and necessary.