timedated: log about unit enablement actions and enhance logs in general

timedated: log about unit enablement actions and enhance logs in general

The general idea is that for things which can occur repeatedly, like SetNTP
being called with the same argument, we only log at debug level. For things
which change state, like when we realize that a unit that wasn't enabled
before needs to be enabled, we log at info level.

Also, don't return success if there are no units loaded.