New with_path_overlay launcher doesn't clean up

[jaraco]

Q: When does a with context never exit?
A: When the command in the context invokes execv.

It occurred to me today that using execve to launch the child process would eliminate the cleanup logic for the temporary directory.

I started to try to think of some clever way to cause the cleanup logic to be invoked after the child process has exited:

• somehow inject an atexit behavior into the child Python process.
• run a separate subprocess that monitors the parent and cleans up when the parent exits.
• invent an OS-level process-temporary store, a part of the file system that the OS will guarantee will be cleaned up when the process exits. How cool would that be? I'm surprised such a thing doesn't already exist.

Then I realized, I'm trying too hard and I should probably just switch rwt back to the old subprocess behavior.