Arcyd: add rudimentary multiprocessing

Start to move towards a multiprocessing model by delegating all real
work to a worker process shortly after 'arcyd start'.

Immediately try to join the worker process. Propagate SIGTERM to the
worker process.

Note some of the problems that will need to be addressed before spawning
more workers.

Test Plan:

Try to get general coverage
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

Exercise shutdown paths and inspect logs
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    $ tail var/log/*
    $ arcyd start
    $ arcyd stop --force
    $ tail var/log/*