Check presence of required functions in daemon launcher

Summary: I had quite some trouble starting daemons on a machine with installed **pcntl** but with [[ http://www.php.net/ini.core#ini.disable-functions | disabled ]] all functions from it.

Test Plan:
  $ bin/phd start

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5568