Change `Daemon` state handling, rename to "signal"

This renames the `Daemon` state to signal and removes the public
query methods. This is a better name because the value represents
the last command or signal that was sent to the daemon. It doesn't
reflect it's current state. The public query methods of the
daemon's state are not needed outside of the daemon, they are only
used internally. The state of the daemon can be determined via the
`running?` boolean.