bus: introduce concept of a default bus for each thread and make use of it everywhere

We want to emphasize bus connections as per-thread communication
primitives, hence introduce a concept of a per-thread default bus, and
make use of it everywhere.

src/nspawn/nspawn.c

@@ -965,7 +965,7 @@ static int terminate_machine(pid_t pid) {
         const char *path;
         int r;
 
-        r = sd_bus_open_system(&bus);
+        r = sd_bus_default_system(&bus);
         if (r < 0) {
                 log_error("Failed to open system bus: %s", strerror(-r));
                 return r;