Add a setup issue to detect systems vulnerable to "Shellshock"

Summary: Ref T6185. Although it seems that we can't easily defuse or mitigate this, we can at least warn administrators.

Test Plan: Ran on my (unpatched, local) system, got a setup warning.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6185

Differential Revision: https://secure.phabricator.com/D10561