Support receiving PROXY protocol version 1 and 2.

Support receiving PROXY protocol version 1 and 2.

PROXY protocol has been developed by Willy Tarreau of HAProxy for
communicating original src and dst IP:port details between proxies and
load balancers in a protocol-agnostic way.

stunnel, HAProxy and some other HTTP proxying software are already
enabled and by adding support to Squid we can effectively chain these
proxies without having to rely on X-Forwarded-For headers.

This patch adds http_port mode flag (require-proxy-header) to signal the
protocol is in use, parsing and processing logics for the PROXY protocol
headers on new connections, and the proxy_protocol_access control to
manage inbound connections.
 The indirect client security/trust model remains unchanged. As do all
HTTP related logics on the connection once PROXY protocol header has
been received.

Furture Work:
 * support sending PROXY protocol to cache_peers
 * support receiving PROXY protocol on https_port
 * rework the PROXY parse logics as a Parser-NG child parser.