CMS_NODE_API.available_parents_for_node

- Fixed comment.
     - added check to know if there are potencial cycles
     - added postcondition to ensure the list of potencial parent will not produce a cycle.

CMS_PAGE_NODE_TYPE_WEBFORM_MANAGER.update_node
     - Updated code to handle unassing parent from a node when the user
     - submit a -1 value.

NODE_FORM_RESPONSE.edit_form_validation
      - Added validation for node_parent, check if the input is valid and then
      - if exist a valid node in the list of available parents for the current node.

Signed-off-by: jvelilla <javier.hector@gmail.com>

modules/node/handler/node_handler.e

@@ -103,7 +103,7 @@ feature -- HTTP Methods
 					-- Add child node
 				l_nid := node_id_path_parameter (req)
 				if l_nid > 0 then
-						-- create a new child node with node id l_id as parent.
+						-- create a new child node with node id `l_id' as parent.
 					create_new_node (req, res)
 				else
 					send_not_found (req, res)