linking: handle 'node.linger' property when target node not known

Do not send an error to the client when the target is not defined
and the 'node.linger' property is set.

It is not absolutely necessary that every node has a defined target.

We can have a 'Stream/Output/*' node which can be linked to
multiple 'Stream/Input/*' nodes and only the 'Stream/Input/*'
nodes have a defined target.

src/scripts/linking/prepare-link.lua

@@ -104,6 +104,13 @@ SimpleEventHook {
         return
       end
 
+      local linger = cutils.parseBool (si_props ["node.linger"])
+
+      if linger then
+        log:info (si, "... node linger")
+        return
+      end
+
       lutils.sendClientError (event, node,
           reconnect and "no target node available" or "target not found")