ipykernel messes with signal handlers

[jdemeyer]

This is a follow-up of #49. That PR added support for restoring a custom SIGINT handler. This was used by the SageMath kernel to restore its custom SIGINT handler. However, this really should be made more general.

Given that upstream Python does not support restoring arbitrary signal handlers (see https://bugs.python.org/issue13285 for that), I would like to add optional support for cysignals.pysignals in the IPython kernel.