inputhook: disable CTRL+C when a hook is active. On systems with 'readline', it's very likely to intercept a signal during a select() call. The default SIGINT handler will schedule a KeyboardInterrupt exception to be raised as soon as possible. If ctypes is used to install a Python callback for PyOS_InputHook, this will happen as soon as the bytecode execution starts, so even if the first instruction of the callback is a `try: ... except KeyboardInterrupt` clause, it's actually too late. As ctypes doesn't allow a Python callback to raise an exception, this ends up with IPython detecting an internal error... not pretty. We must therefore ignore the SIGINT signals until we are sure the exception handler is active, in the Python callback.