v0.7 Replace prctl dependency with ctypes, and improve error handling with PID files

Our prior use of prctl calls for setting the process death signal (and setting process names in testing) used python-prctl. However, that library is GPL-licensed, and this library is Apache 2.0-licensed. Switched to using our own internal ctypes-based method for those calls.

Also, if a child process didn't start up, and we were expecting it to write a PID file, we had an error reading the non-existent file. Switched to handle this more sensibly and give appropriate explanations.