sd-bus: introduce sd_bus_slot objects encapsulating callbacks or vtables attached to a bus connection

This makes callback behaviour more like sd-event or sd-resolve, and
creates proper object for unregistering callbacks.

Taking the refernce to the slot is optional. If not taken life time of
the slot will be bound to the underlying bus object (or in the case of
an async call until the reply has been recieved).

src/network/networkd-link.c

@@ -684,7 +684,7 @@ static int link_set_hostname(Link *link, const char *hostname) {
         if (r < 0)
                 return r;
 
-        r = sd_bus_call_async(link->manager->bus, m, set_hostname_handler, link, 0, NULL);
+        r = sd_bus_call_async(link->manager->bus, NULL, m, set_hostname_handler, link, 0);
         if (r < 0)
                 log_error_link(link, "Could not set transient hostname: %s", strerror(-r));