Summary:
We have a lot of mess to get through before we can load libphutil and enter Phabricator code properly. Move it to a dedicated class.
I'm probably going to merge PhabricatorRequestOverseer into this, although the check that lives there now is kind of weird. It also does not really need to be a pre-load check and could be handled better.
I stopped shoving stuff in here once I got to ENV stuff, I'm going to tackle that next.
Test Plan: Ran phabricator normally; introduced fatals and misconfigurations. Grepped for changed symbols.
Reviewers: btrahan, vrana
Reviewed By: btrahan
CC: aran, asherkin
Maniphest Tasks: T2223
Differential Revision: https://secure.phabricator.com/D4282
