Move ob_get_level() script initialization check to libphutil from arcanist

Summary: See D1950, D1951. We should always normalize any output buffering initialized in the environmen when running scripts.

Test Plan: Ran "arc".

Reviewers: btrahan, killermonk

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1952