Don't rely on include_path to include files.

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

props ketwaroo, hakre.
fixes #17092.

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Clean up the path calculations in wp::parse_request().

props evansolomon for initial cleanup.
fixes #22209.

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'sanitize_user' is a filter, not an action. fixes #25411.
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Simplify the login page viewport meta for mobile devices, so it's less restrictive on the user. Allows for developers to override if necessary via the `login_head` action. props azaozz. fixes #24777.

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Better error messaging on install.php. props DrewAPicture, ocean90. fixes #24982.

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