Features:
+* add API to clone sd_bus_message objects
+
* sd-bus: synthesized messages should get serial number (uint32_t) -1
* sd-event: allow multiple signal handlers per signal
* sd-bus: enforce signatures on response messages
-* sd-bus: make message handlers take an sd_bus_error and generate error replies automatically if they are set
-
* sd-bus: see if we can drop more message validation on the sending side
* sd-bus: introduce sd_bus_creds object and attach it to messages as well as allow querying it for names