Don't send SIGINT to Py4J gateway subprocess.

This addresses SPARK-885, a usability issue where PySpark's
Java gateway process would be killed if the user hit ctrl-c.

Note that SIGINT still won't cancel the running s

This fix is based on http://stackoverflow.com/questions/5045771
Merge pull request #870 from JoshRosen/spark-885

Don't send SIGINT / ctrl-c to Py4J gateway subprocess