Generate URI's using a configured canonical base url

The attached patch allows optionally overriding $base_url for purposes of Pathologic's filtration process. The intention is to use pathologic to make sure internal page URLs go to a particular canonical version which may not be based on the global base_url parameter.

This leverages an optional parameter of the url() function to override it's use of the global $base_url. This parameter is marked as being for internal use.

Use Case

• Domain A is public facing, http-only.
• Domain B has SSL support
• Authenticated traffic is only site staff & administrators.
• Redirect all authenticated traffic to Domain B (with path-based multi-site) for security
• Enforce a canonical URL on content created via the "administrative domain" so text render cache is not in a race condition between the use of the two different domains.

I have linked to #2071695: local URLs don't work when Drupal is in a subdirectory as a related issue because of the coincidence that my use case happens to use path-based multisite, but that relates more to the scale on which I am using the functionality requested, not to the functionality itself.