R1:772a94236644

Detect 'post_max_size' more robustly

Summary:
Currently, when a user runs "arc diff" and the diff exceeds PHP's 'post_max_size', they get a very confusing and irrelevant error about a missing Conduit session token. The reason for this is that 'post_max_size' doesn't build $_POST, so //all// the data is missing.

We try to detect this, but currently only do so effectively for specific file upload forms. Broaden the detection to cover all cases.

Previously, we ran into an issue where Firefox + HTML5 drag…