(stable) Parse "multipart/form-data" bodies even if "enable_post_data_reading" is on

Summary:
Ref T4369. During T13507, I set my "max_post_size" to a very small value, like 7 (i.e., 7 bytes). This essentially disables "enable_post_data_reading" even if the setting is technically on.

This breaks forms which use "multipart/form-data", which are rare but not nonexistent. Notably, forms in Config use this setting (because of ui.header stuff?) although perhaps they should not or no longer need to.

This can be fixed by parsing the raw input.

Since the only reason we don't parse the raw input is concern that we may not be able to read it (per documentation, but never actually observed), and we do a strlen() test anyway, just read it unconditionally.

This should fix cases where POST data wasn't read because of "max_post_size" without impacting anything else.

Test Plan: With very small "max_post_size", updated "ui.footer-items" in Config. Before: form acted as a no-op. After: form submitted.

Maniphest Tasks: T4369

Differential Revision: https://secure.phabricator.com/D21165