Arcyd: death by killfile is not an error

The regular way to shut down the daemon is via the killfile, treat it as
an info event instead of an error and don't send emails about it.

Test Plan:

Update docs, run the cheap tests for good measure
$ ./precommit.sh

Exercise the new code manually to verify
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    $ cat var/log/info
    $ cat var/log/stderr
    .. note the lack of exceptions, note logged as info ..