Provide proper AJAX URL resp. base path depending on context

Issue Summary

admin_menu 3.x uses an AJAX/AHAH callback to fetch the menu from the server. Now, I'm about to commit yet another patch to admin_menu to fix an annoying problem of wrong URLs in JavaScript callbacks.

The point is: Whenever you retrieve raw data, such as perhaps a user list for autocompletion, then you do not run into this problem. But whenever you try to retrieve data/content that

a) may be translated or localized

b) may be access-restricted (like node access, e.g. Domain Access)

then Drupal.settings.basePath does not work out at all, because it does not account for any language (path prefix or domain) or any absolute domain/multi-site/section URL components.

I'm running out of ideas, and we badly need to fix this.

Problem: There are countless of things that affect the proper base path for AJAX/AHAH requests.

• Depending on Clean URLs, you need to use /?q=foo or /foo. (to be continued below)
• Depending on the base path, you need to use / or my/drupal/.
• Depending on the language system configuration, you need to use /foo or /de/foo. Depending on the language negotiation configuration, the path prefix decides, in which language any content is (or should be) returned.
• Depending on any custom_url_rewrite() and custom_url_rewrite_outbound() functions, you possibly need to use /foo?custom=query. Such additional URL components may be used by modules to provide further context, which in turn, may or may not be used to grant more or less privileges to the current user. (also node_access related)
• When Clean URLs are disabled, url('') returns / (front page), i.e. no ?q= at all.
• When Clean URLs are enabled, url('') returns a trailing slash, but url('foo') returns no trailing slash.

Site note: Drupal.settings.basePath does not account for anything in this list. Which is probably OK for all callbacks that return raw data only (i.e. no content).