Arcyd: BREAK, remove 'arcyd stop --force'

Remove the '--force' flag from 'arcyd stop' as it's going to take extra
work to implement for a multithreading environment.

The intention is to make 'arcyd stop' work by sending 'SIGTERM' anyway
and handle that more gracefully.

Update 'test_shell' to use the regular killfile path.

Test Plan:
$ ./precommit.sh

Exercise the changed script
$ ./testbed/arcyd/test_shell.sh
    $ exit
Arcyd: add rudimentary multithreading

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

Immediately try to join the worker thread.

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/*