sd-bus: introduce new sd_bus_default_flush_close() call

If code enqueues a message on one of the default busses, but doesn't
sync on it, and immediately drops the reference to the bus again, it
will stay queued and consume memory. Intrdouce a new call
sd_bus_default_flush_close() that can be invoked at the end of programs
(or threads) and flushes out all unsent messages on any of the default
busses.

src/libsystemd/libsystemd.sym

@@ -473,3 +473,8 @@ global:
         sd_pid_get_cgroup;
         sd_peer_get_cgroup;
 } LIBSYSTEMD_222;
+
+LIBSYSTEMD_227 {
+global:
+        sd_bus_default_flush_close;
+} LIBSYSTEMD_226;