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
'sanitize_user' is a filter, not an action. fixes #25411.
Built from https://develop.svn.wordpress.org/trunk@25618


git-svn-id: http://core.svn.wordpress.org/trunk@25535 1a063a9b-81f0-0310-95a4-ce76da25c4cd