Don't rely on include_path to include files.

Always use dirname() or, once available, ABSPATH.

props ketwaroo, hakre.
fixes #17092.

Built from https://develop.svn.wordpress.org/trunk@25616


git-svn-id: http://core.svn.wordpress.org/trunk@25533 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Clean up the path calculations in wp::parse_request().

props evansolomon for initial cleanup.
fixes #22209.

Built from https://develop.svn.wordpress.org/trunk@25617


git-svn-id: http://core.svn.wordpress.org/trunk@25534 1a063a9b-81f0-0310-95a4-ce76da25c4cd