Sets up all the relevant information for the Admin Search functions

• Requires the admin_forum permission
• Accessed via .../index.php?action=admin;area=search[;search_type=x]
• Sets up an array of applicable sub-actions (search types) and the function that goes with each
• Search type specified by "search_type" request variable (either from a form or from the query string)
• Defaults to 'internal'
• Calls the appropriate function based on the search_type