Disable cURL protocols other than HTTP and HTTPS

Summary:
See here for lols:

http://blog.volema.com/curl-rce.html

Although I'm not terribly worried about someone actually building a payload for this and mounting an attack, the underlying behavior is very silly. Shut this class of things down so that we never have to debug arc raising FTP errors, if nothing else.

Test Plan: Ran arc network commands.

Reviewers: btrahan, vrana, indiefan

Reviewed By: indiefan

CC: indiefan, aran

Differential Revision: https://secure.phabricator.com/D4864