M run.py

1. Move subprocess socket handling to a subthread - "SockThread".
2. In the subprocess, implement a queue and global completion and exit
flags.  Execute code after it is passed through the queue.  (Currently,
user code is executed in SockThread.  The next phase of development will
move the tail of the queue to MainThread.)
3. Implement an RPC message used to shut down the execution server.
4. Improve normal and exception subprocess exits.

(At this checkin a "pass loop" interrupt doesn't work on any platform.  It
will be restored for all platforms once user code execution is moved to
MainThread.)