$context['user_agent'] was only being respected if
other headers happened to be getting set as well.
Even then, the header being set was "User_Agent" rather than
the correct "User-Agent" version.
This change is mostly for FB internal organizational reasons.
Building is not effected beyond the fact that the target now
lands in hphp/hhvm/hhvm rather than src/hhvm/hhvm.