resolved: log process info of clients requesting resolution via D-Bus

resolved: log process info of clients requesting resolution via D-Bus

Let's make things more debuggable: when debug logging is on, let's
say which client is asking for our services.

This is helpful for easily figuring out which local process might
interfere with your debugging sessions by issuing additional requests
while you try to debug a request (I am looking at you, geoclue!).