MINOR: config: warn when a server with no specific port uses rdp-cookie

MINOR: config: warn when a server with no specific port uses rdp-cookie

Mathew Levett reported an issue which is a bit nasty and hard to track
down. RDP cookies contain both the IP and the port, and haproxy matches
them exactly. So if a server has no port specified (or a remapped port),
it will never match a port specified in a cookie. Better warn the user
when this is detected.