Allow Phabricator to run with "enable_post_data_reading" disabled

Summary:
Ref T13008. Depends on D18701. The overall goal here is to make turning enable_post_data_reading off not break things, so we can run rate limiting checks before we read file uploads.

The biggest blocker for this is that turning it off stops $_FILES from coming into existence.

This appears to mostly work. Specifically:

• Skip the max_post_size check when POST is off, since it's meaningless.
• Don't read or scrub $_POST at startup when POST is off.
• When we rebuild REQUEST and POST before processing requests, do multipart parsing if we need to and rebuild FILES.
• Skip the is_uploaded_file() check if we built FILES ourselves.

This probably breaks a couple of small things, like maybe __profile__ and other DarkConsole triggers over POST, and probably some other weird stuff. The parsers may also need more work than they've received so far.

I also need to verify that this actually works (i.e., lets us run code without reading the request body) but I'll include that in the change where I update the actual rate limiting.

Test Plan:

• Disabled enable_post_data_reading.
• Uploaded a file with a vanilla upload form (project profile image).
• Uploaded a file with drag and drop.
• Used DarkConsole.
• Submitted comments.
• Created a task.
• Browsed around.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13008

Differential Revision: https://secure.phabricator.com/D18702