Arcyd: add 'stop' command

Arcyd is hard to stop gracefully.

Provide 'arcyd stop' to make the options discoverable and documented.

Test Plan:
Use 'arcyd stop' where appropriate in ./testbed/arcyd/exercise_arcyd.sh
$ ./precommit.sh
testbed/arcyd/test_shell.sh: use new cli

Convert test_shell.sh to use the new Arcyd CLI

Test Plan:
$ ./testbed/arcyd/test_shell.sh
WIP: Arcyd: use pidfile to improve start/stop