Stream "multipart/form-data" file uploads to disk

Summary: Depends on D18700. Ref T13008. Adds support for sending file uploads to disk, similar to the buitin PHP mechanism, and a compatibility layer for building $_FILES.

Test Plan: See next diff; uploaded files using a vanilla file upload with enable_post_data_reading off.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13008

Differential Revision: https://secure.phabricator.com/D18701