Provide "phutil_build_http_querystring()", like "http_build_query()" but not wrong by default

Summary:
See D20039. Although http_build_query() works fine for the most part, it does the wrong thing by default if arg_separator.output is set in configuration.

Provide a wrapper which forces the expected behavior.

Test Plan: Ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20049