MINOR: listener: introduce listener_backlog() to report the backlog value

In an attempt to try to provide automatic maxconn settings, we need to
decorrelate a listner's backlog and maxconn so that these values can be
independent. This introduces a listener_backlog() function which retrieves
the backlog value from the listener's backlog, the frontend's, the
listener's maxconn, the frontend's or falls back to 1024. This
corresponds to what was done in cfgparse.c to force a value there except
the last fallback which was not set since the frontend's maxconn is always
known.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index d454d4c..980a226 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -315,7 +315,7 @@
 		ready = 0;
 
 	if (!(ext && ready) && /* only listen if not already done by external process */
-	    listen(fd, listener->backlog ? listener->backlog : listener->maxconn) < 0) {
+	    listen(fd, listener_backlog(listener)) < 0) {
 		err |= ERR_FATAL | ERR_ALERT;
 		msg = "cannot listen to UNIX socket";
 		goto err_unlink_temp;