Merge pull request #4260 from takluyver/system-raw-subprocess Use subprocess for system_raw This unifies the exit codes from system_raw and system_piped to use the subprocess model on Unix - positive numbers for exit status, negative for terminating signal. This doesn't change the situation on Windows. system_raw will give the raw exit code (Windows exit statuses don't appear to include the terminating signal), and system_piped will give nothing - our process_win32 machinery never gets the exit code, and I don't want to go messing around with that.