dbus: support triggering graceful agent reset, configuration reload

[lucab]

For the usecase described in #671 (comment), it would be useful to expose a DBus method to allow external consumers to request a graceful agent reset (e.g in order to switch to a different configuration).

In general it isn't always possible to gracefully reset, for example it may not be allowed when the agent is already finalizing an update.
Thus we first need to identify valid reset states/points, and intermix the reset signal with the existing state-machine ticking.