Whitelist blacklisting pcntl_ functions for setup checks so Debian installs don't fatal instantly

Summary: See IRC. This is dumb but I think we should try to work by default on Debian, and it doesn't cost us too much. See inline comment for more.

Test Plan:

• No disable_functions, restarted, worked fine.
• Set disable_functions = pcntl_derp, restarted, worked fine.
• Set disable_functions = derp, restarted, setup fatal.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6741