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