Don't leak $root when initializing libphutil scripts

Summary: We leak the global variable $root into the including context, which can ruin things. Put the whole thing in a function.

Test Plan: Ran arc, mail_handler.php.

Reviewers: Makinde, btrahan

Reviewed By: Makinde

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1997