Disable XML entity loader by default in libphutil

Summary:
See <https://www.facebook.com/BugBounty/posts/778897822124446> and <http://www.ubercomp.com/posts/2014-01-16_facebook_remote_code_execution>.

By default, `SimpleXMLElement` will pretty much just run whatever code you want when handed a specially crafted document. We currently load XML only from trusted (S3, EC2, git/svn/hg) or local sources (unit test runners) so there are no concrete vulnerabilities in Phabricator, but this behavior is incredibly dangerous, surprising, and highly undesirable.

Test Plan: There's an example of a document which does bad things on <http://www.php.net/manual/en/function.libxml-disable-entity-loader.php>. I verified that `SimpleXMLElement` reads `/etc/passwd` when handed this document, then applied the fix. It no longer reads arbitrary files off disk.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8049