timedated: use libsystemd-bus instead of libdbus for bus communication

Among other things this also adds a few things necessary for the change:

- Considerably more powerful error returning APIs in libsystemd-bus

- Adapter for connecting an sd_bus to an sd_event

- As I reworked the PolicyKit logic to the new library I also made it
  asynchronous, so that PolicyKit requests of one user cannot block out
  another user anymore.

- We always use the macro names for common bus error. That way it is
  harder to mistype them since the compiler will notice

src/stdio-bridge/stdio-bridge.c

@@ -36,6 +36,7 @@
 #include "sd-bus.h"
 #include "bus-internal.h"
 #include "bus-message.h"
+#include "bus-util.h"
 
 int main(int argc, char *argv[]) {
         _cleanup_bus_unref_ sd_bus *a = NULL, *b = NULL;