PROXY protocol support

PFA a simple patch that implements support for the PROXY protocol.

This is a protocol common and very light weight in proxies and load
balancers (haproxy is one common example, but also for example the AWS
cloud load balancers). Basically this protocol prefixes the normal
connection with a header and a specification of what the original host
was, allowing the server to unwrap that and get the correct client
address instead of just the proxy ip address. It is a one-way protocol
in that there is no response from the server, it's just purely a
prefix of the IP information.

Using this when PostgreSQL is behind a proxy allows us to keep using
pg_hba.conf rules based on the original ip address, as well as track
the original address in log messages and pg_stat_activity etc.

The implementation adds a parameter named proxy_servers which lists
the ips or ip+cidr mask to be trusted. Since a proxy can decide what
the origin is, and this is used for security decisions, it's very
important to not just trust any server, only those that are
intentionally used. By default, no servers are listed, and thus the
protocol is disabled.

When specified, and the connection on the normal port has the proxy
prefix on it, and the connection comes in from one of the addresses
listed as valid proxy servers, we will replace the actual IP address
of the client with the one specified in the proxy packet.

Currently there is no information about the proxy server in the
pg_stat_activity view, it's only available as a log message. But maybe
it should go in pg_stat_activity as well? Or in a separate
pg_stat_proxy view?

(In passing, I note that pq_discardbytes were in pqcomm.h, yet listed
as static in pqcomm.c -- but now made non-static)

--
Magnus Hagander
Me: https://www.hagander.net/
Work: https://www.redpill-linpro.com/