Missing hook_theme() implementation.

Ran into an issue with link_node on a project when trying to override node_links output: while link_node has theme functions, it does not implement hook_theme() and it does not invoke theme() to render output, making it impossible to override those theme functions.

Attached is a simple patch that makes the following changes:

• implements hook_theme()
• changes the invocations of theme functions to use theme()
• a minor clean-up in theme_link_node_thumbnail (remove unused varaible)
• renamed theme_link_node_thumbnail to theme_link_node_link, since thumbnail only makes sense when linking to nodes of type image. Since modules and themes could not override link_node's theme functions prior to this patch, this seems a good time to make sure the function name correctly reflects its use.