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

Author: vrana

scripts/daemon/phabricator_daemon_launcher.php

@@ -15,6 +15,16 @@ function must_have_extension($ext) {
          "install it to run daemons on this machine.\n";
     exit(1);
   }
+
+  $extension = new ReflectionExtension($ext);
+  foreach ($extension->getFunctions() as $function) {
+    $function = $function->name;
+    if (!function_exists($function)) {
+      echo "ERROR: The PHP function {$function}() is disabled. You must ".
+           "enable it to run daemons on this machine.\n";
+      exit(1);
+    }
+  }
 }
 
 $command = isset($argv[1]) ? $argv[1] : 'help';