Signal handling doesn't work in interactive Python session

Currently, to use the scarab-based signal handling you have to create a signal_handler and add the appropriate cancelable. This is fine for executables, e.g. dl-serve. However, it's not workable for an interactive session.

I created a script that uses `core.Interface` to e.g. `get peaches` from the KVS. For network reasons it wasn't able to contact the broker, so it tried multiple times using exponential backoff. I attempted to cancel that with ctrl-c, but it didn't work because the signal hadn't been setup to be caught. I got it to stop by repeating ctrl-c twice, resulting in an unhandled exception and a long traceback.

I'm not sure what the right way to handle this is, at the moment.

Start at core.Interface and compare its signal_handler and cancelable setup with the dl-serve executable path. Reproduce the broker retry scenario in an interactive Python session and verify that Ctrl-C cancels cleanly without an unhandled exception or long traceback; the issue names no file or test.

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