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

This addresses [SPARK-885](https://spark-project.atlassian.net/browse/SPARK-885), a usability issue where PySpark's Java gateway process would be killed if the user hit ctrl-c, preventing new jobs from being run.  This behavior was annoying because I tend to reflexively hit ctrl-c if I want to start a blank input line.

This is based on an answer to http://stackoverflow.com/questions/5045771.