Add some sytem locale utilities to libphutil
Ref T13060. Ref T7339. Ref T5554. Although I don't plan to pursue resolving the latter two issues for now, I think we do have enough information to be confident that setlocale() is the ultimate pathway forward.
Test Plan: Ran unit tests, got a pass.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18990