sd-bus: add new sd_bus_is_ready() API

This new call is much light sd_bus_is_open(), but returns true only if
the connection is fully set up, i.e. after we finished with the
authentication and Hello() phase. This API is useful for clients in
particular when using the "watch_bind" feature, as that way it can be
determined in advance whether it makes sense to sync on some operation.

src/libsystemd/libsystemd.sym

@@ -540,4 +540,5 @@ global:
         sd_bus_add_match_async;
         sd_bus_match_signal;
         sd_bus_match_signal_async;
+        sd_bus_is_ready;
 } LIBSYSTEMD_236;

src/libsystemd/sd-bus/sd-bus.c

@@ -1396,6 +1396,13 @@ _public_ int sd_bus_is_open(sd_bus *bus) {
         return BUS_IS_OPEN(bus->state);
 }
 
+_public_ int sd_bus_is_ready(sd_bus *bus) {
+        assert_return(bus, -EINVAL);
+        assert_return(!bus_pid_changed(bus), -ECHILD);
+
+        return bus->state == BUS_RUNNING;
+}
+
 _public_ int sd_bus_can_send(sd_bus *bus, char type) {
         int r;