Add function for deleting menus and menu links

This is untested, but basically copy/paste/modify of http://api.drupal.org/api/function/menu_delete_menu_confirm_submit/6.

Oops. Noticed one typo already.

Now with fewer typos! This actually seems to work here. I'm not sure what this should be returning, though.

Additionally, it probably makes sense to break this up into a install_menu_delete_menu_item function and call it from here.

Well, interesting. So _menu_delete_link() does this totally bizarre behaviour of re-parenting children to where their parent was when it's deleted. So I think we need another function here, on the order of http://api.drupal.org/api/function/taxonomy_del_term/6, that goes through and recursively deletes all children menu links.

Now with install_menu_remove_menu_link_recursive() :)

And now with matching cache clearage at the end.