url-downloader: further fixes

squid 3.1 that comes bundled with precise is different than 3.3 that
comes bundled with trusty in quite a few ways. Aside from the ACL all
that is already built-in in 3.1, 3.3 also has builtin manager, localhost
and to_localhost ACLs. For now target precise. Full trusty support
should be done better in a squid module that will abstract of all this.
broken_vary_encoding is another example of this. Supported in 2.x,
unsupported in 3.1, undetermined in 3.3, as in it is accepted in the
configuration file but documentation does not state it as a legal
configuration directive

Change-Id: Ie9edff95487762f8587218d414837b65c29cb038
Merge "url-downloader: further fixes" into production