Improved UBus supported

- aligned the handling of UBus connections with the DBus code as it
makes it a bit easier to comprehend;
- added logging to the various UBus calls to aid debugging from an
enduser point of view, but be careful to not flood the logs;
- show the (lack of) support for UBus in the configuration string.

src/config.h

@@ -362,6 +362,10 @@ static char *compile_opts =
 "no-"
 #endif
 "DBus "
+#ifndef HAVE_UBUS
+"no-"
+#endif
+"UBus "
 #ifndef LOCALEDIR
 "no-"
 #endif