R1:6a69b4699ecb

file.upload set policy explicitly

Summary:
This is pretty basic allowing a user to set the
policy as a valid string ('no-one' or 'users') or
as a valid PHID. Without an explicit policy
a permissive one is set.

Test Plan:
Tested using the python-phabricator module (very basic api wrapper).

The arc cli syntax was evading me.

```import base64
from phabricator import Phabricator
phab = Phabricator()
with open('mypic.jpg') as f:
encoded = base64.b64encode(f.read())

//set no-one as viewer which really…