Turns out multiprocessing already converts retcode

Thanks, Python.

http://en.wikipedia.org/wiki/Principle_of_least_astonishment
Fixes LogReceived#join()
Fixes test for POSIX

"Signal" is printed instead of "Exception".
Adds a ack_fifo

Writing on the fifo then crashing, we can't be sure if the remote will
be able to read before the connection is torn down. This creates a
feedback connection so we can know for sure the message was received
before going on executing.
Of course, this slows things down. It is also poorly written. But tests
pass.