Option to update existing nodes with same title

This patch changes the "Titles must be unique for this node type" checkbox in supported/node.inc to a select box with the options:
- Don't import
- Create new node
- Update existing node
And description:
Select Don't import if you don't want to import nodes which have the same title as an existing node, Create new node if you want all imported nodes to be created as new nodes, or Update existing node if you want existing nodes with the same title as nodes to import to be updated with the imported node data, creating a new revision for those nodes (if more than one node with the same title and of the same type is found none of them will be updated).

The update existing nodes option is handled by setting the node object nid field to that of the existing nodes nid (if there's only one with the same title and type, otherwise an error is generated). This is then picked up by save_node which updates the existing node instead of creating a new one.