sd-bus: add a "recursive" mode to sd_bus_track

This adds an optional "recursive" counting mode to sd_bus_track. If enabled
adding the same name multiple times to an sd_bus_track object is counted
individually, so that it also has to be removed the same number of times before
it is gone again from the tracking object.

This functionality is useful for implementing local ref counted objects that
peers make take references on.

src/libsystemd/libsystemd.sym

@@ -500,3 +500,11 @@ LIBSYSTEMD_231 {
 global:
         sd_event_get_iteration;
 } LIBSYSTEMD_230;
+
+LIBSYSTEMD_232 {
+global:
+        sd_bus_track_set_recursive;
+        sd_bus_track_get_recursive;
+        sd_bus_track_count_name;
+        sd_bus_track_count_sender;
+} LIBSYSTEMD_231;