[META] Re-architect use of menus, tabs, and forms in Search module

As things are now, the way Search uses the form system and the menu/tab system is non-standard, and it tends to make things rather difficult. For instance, there is no clear distinction in the URLs between search tabs/modules and search terms (e.g. path search/foo could mean you are searching for "foo" or that you are using the "foo" tab).

This leads to things like these issues, some of which may have been partially fixed or worked around to some extent in Drupal 7:

• #751850: searching for ldap throws error
• #600424: search.module pulls arguments directly from $_GET rather than using the menu system
• #245103: Search page tabs not highlighting
• #497206: Avoid search conflicts with other forms, use menu API instead of search_get_keys() (this might not actually be related, but I think it is)
• #600392: search_form() has an $action argument but this argument is partially ignored
• #518512: Search from block with no keywords doesn't display the "Please enter some keywords" message

Possibly we should scrap the idea of each search module being a tab, and instead be using radio buttons, and using the standard form submit mechanisms of Drupal? It needs some thought...