ACTION-390: alternative UA affordances for DNT choice

Peter asked me to assemble some examples of User Agents offering different
types of affordances for DNT choice.

[First, for those with less experience in web standards, let's review the
definition of "User Agent".   The TPE spec includes a standard definition:
"This specification uses the term user agent to refer to any of the various
client programs capable of initiating HTTP requests, including, but not
limited to, browsers, spiders (web-based robots), command-line tools,
native applications, and mobile apps
[HTTP11<http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html#bib-HTTP11>]."
   HTTP requests are used for many purposes beyond loading HTML pages for
display in browsers.  Although we might be tempted to think of "User Agent"
as synonymous with "browser," there are many UAs that are not browsers.]

User Agent functionality is built into many types of consumer electronics
or "smart object" products, including alarm clocks, pedometers, audio
players, car electronics, bathroom scales, and even dog collars.   (The
collar reports your dog's location over time.)   These are not
hypotheticals; they are all real products on the market.   Many products of
this type do no offer a rich user interface on the device itself.  Instead,
they offer control and interaction via a web site provided separately from
the product itself, which the consumer accesses using their ordinary
desktop browser.

For example, a music player device might offer the ability to subscribe to
podcasts, with the device automatically downloading new podcast episodes
from subscribed-to podcasts as they become available.   When downloading a
new podcast episode, the player device would be acting as a user agent
(initiating an HTTP request).  Yet the player device might not offer a user
interface with rich controls.  Instead, the user might set up and control
their podcast subscriptions via an external website affiliated with the
device.

In this case, it is possible to offer the user DNT choice via the external
website.  But note that this choice would not be offered through the user
agent (the music player device) itself---and the external website is not a
user agent at all.  Therefore a spec that required choice to be offered
*directly by* the user agent would not be implementable in this scenario,
while one that merely required clear choice *with respect to* the user
agent would be implementable for this type of user agent.

Another type of UA that can't offer a direct DNT affordance to the user is
a service that acts asynchronously on the user's behalf.   One example is
Ifttt.   You tell Ifttt a "recipe" such as "rebroadcast all of my Twitter
tweets as Facebook wall posts" or "clip any Facebook photo tagged with my
name and upload it into Evernote", etc.   Then Ifttt periodically accesses
the various sites on your behalf to carry out the recipes.  When Ifttt
accesses these sites, it is acting as a User Agent, but you are not present
and this UA doesn't offer you a direct user interface.   You can control
the status of your Ifttt account via an external control panel, which is
not a User Agent.   Again, notification and choice are possible *for* the
Ifttt User Agent, but not *through* the User Agent itself.

This should give an idea of some of the scenarios that can come up.   There
are others that pose different challenges, such as command-line tools, or
tools that user HTTP "in the background" to update code or data in an app,
or code that isn't allowed to offer a rich user interface for security
reasons.   (A rich UI can be used, e.g., to trick the user into entering a
sensitive password, so some systems block less-trusted code from displaying
a rich or large UI.)