sd-varlink: fix bug when enqueuing messages with fds asynchronously

When determining the poll events to wait for we need to take the queue
of pending messages that carry fds into account. Otherwise we might end
up not waking up if such an fd-carrying message is enqueued
asynchronously (i.e. not from a dispatch callback).

(cherry picked from commit 7b4b3a8)
(cherry picked from commit b2751b9)
(cherry picked from commit 03d691f)

Author: poettering

src/shared/varlink.c

@@ -1298,7 +1298,8 @@ int varlink_get_events(Varlink *v) {
                 ret |= EPOLLIN;
 
         if (!v->write_disconnected &&
-            v->output_buffer_size > 0)
+            (v->output_queue ||
+             v->output_buffer_size > 0))
                 ret |= EPOLLOUT;
 
         return ret;