re-raise KeyboardInterrupt in raw_input

shortens traceback when KeyboardInterrupt is caught deep within zmq.

closes #3786
catch KeyboardInterrupt in debug

avoids traceback for ^C in ipdb.

also prevents KeyboardInterrupt from exiting debugger.
Is this actually desirable?
Merge pull request #3787 from minrk/raw_input_sigint

re-raise KeyboardInterrupt in raw_input

closes #3786