Add a setup warning for probable misconfiguration of 'apc.stat'

Summary: Fixes T3501. apc.stat should generally be 0 in production and 1 in development. Raise a setup warning if it isn't.

Test Plan:
Hit both setup warnings.

{F49176}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3501

Differential Revision: https://secure.phabricator.com/D6376