Edit the search method and search index used.

• Edit the search method and search index used.
• Called by ?action=admin;area=managesearch;sa=method.
• Requires the admin_forum permission.
• Uses the 'select_search_method' sub template of the ManageSearch template.
• Allows to create and delete a fulltext index on the messages table (if supported - currently only supported by MySQL)
• Allows to delete a custom index (that CreateMessageIndex() created).
• Calculates the size of the current search indexes in use.
• Pulls in a few additional search-related db functions using db_extend('search')
• Loads up supported search APIs