Raise a setup fatal for 'disable_functions' or 'disable_classes'

Summary:
Fixes T3709. PHP has two configuration options ('disable_functions', 'disable_classes') which allow functions and classes to be blacklisted at runtime.

Since these break things in an unclear way, raise a setup fatal if they are set.

We take a slightly more tailored approach to these in `phd` already, but I'd rather try just saying "no, this is bad" and see if we can get away with it. I suspect we can, and there's no legitimate reason to blacklist functions given that Phabricator must have access to, e.g., `proc_open()`.

Test Plan: {F54058}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3709

Differential Revision: https://secure.phabricator.com/D6739