Summary:
Fixes T13392. If you have 17 load balancers in sequence, Phabricator will receive requests with at least 17 "X-Forwarded-For" components in the header.

We want to select the 17th-from-last element, since prior elements are not trustworthy.

This currently isn't very easy/obvious, and you have to add a kind of sketchy piece of custom code to `preamble.php` to do any "X-Forwarded-For" parsing. Make handling this correctly easier.

Test Plan:
  - Ran unit tests.
  - Configured my local `preamble.php` to call `preamble_trust_x_forwarded_for_header(4)`, then made `/debug/` dump the header and the final value of `REMOTE_ADDR`.

```
$ curl http://local.phacility.com/debug/
<pre>

HTTP_X_FORWARDED_FOR =
   FINAL REMOTE_ADDR = 127.0.0.1
</pre>
```

```
$ curl -H 'X-Forwarded-For: 1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4, 5.5.5.5, 6.6.6.6' http://local.phacility.com/debug/
<pre>

HTTP_X_FORWARDED_FOR = 1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4, 5.5.5.5, 6.6.6.6
   FINAL REMOTE_ADDR = 3.3.3.3
</pre>
```

```
$ curl -H 'X-Forwarded-For: 5.5.5.5, 6.6.6.6' http://local.phacility.com/debug/
<pre>

HTTP_X_FORWARDED_FOR = 5.5.5.5, 6.6.6.6
   FINAL REMOTE_ADDR = 5.5.5.5
</pre>
```

Maniphest Tasks: T13392

Differential Revision: https://secure.phabricator.com/D20785