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