Donation of mod_proxy_protocol from https://github.com/roadrunner2/mod-proxy-protocol

as per https://github.com/roadrunner2/mod-proxy-protocol/issues/1#issuecomment-269064906


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1776069 13f79535-47bb-0310-9956-ffa450edef68