Enable specification of signal daemon

This PR does two things.

7161c3f: Enable config specification of signal_daemon

Previously, there was no mechanism by which users could choose what method the runonce action would use to initiate a run. This caused problems because the default on Posix platforms, when the daemon was found to be running, was to signal the daemon. This method of triggering a run disallowed any custom options to be passed, such as noop or environment.

This commit exposes signal_daemon as a configuration parameter, but does not change the default.

d84305e: Change signal_daemon default from true to false

When signaling the daemon there is no opportunity to invoke customization, such as noop or environment. It may have made sense at one point to signal the daemon by default, but these limitations and no significant downside to simply calling `puppet agent [args]` mean we should not default to using the signaling method. User expectation is that passing noop or environment should work out of the box, and will already be running as a daemon in most scenarios, rendering signal_daemon=true a degraded experience.

tl;dr: don't use a degraded experience by default.