Add ELB proxy protocol support to the HTTP module.

Previously, this was only supported with http-socket.  If you're
using http-socket, you probably have a webserver on top of you--and if
you have that, then uWSGI isn't receiving the proxy protocol anyway.

This copies proxy1_parse directly from proto/http.c, which should
probably be moved to a shared location, but I'm not sure where that
is.