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.