BUG/MEDIUM: option forwardfor if-none doesn't work with some configurations

BUG/MEDIUM: option forwardfor if-none doesn't work with some configurations

When "option forwardfor" is enabled in a frontend that uses backends,
"if-none" ignores the header name provided in the frontend.
This prevents haproxy to add the X-Forwarded-For header if the option is not
used in the backend.

This may introduce security issues for servers/applications that rely on the
header provided by haproxy.

A minimal configuration which can reproduce the bug:
defaults
	mode http

listen OK
	bind :9000

	option forwardfor if-none
	server s1 127.0.0.1:80

listen BUG-frontend
	bind :9001

	option forwardfor if-none

	default_backend BUG-backend

backend BUG-backend
	server s1 127.0.0.1:80