support chained proxies in mod_extforward
need something like this for lighttpd:
mod_rpaf always uses the last value from the X-Forwarded-For header as
the client IP, breaking when you have several chained proxies (i.e.
Pound in front of Squid in front of Apache).
The attached patch makes it use the last address in the header which is
not in RPAFproxy_ips, falling back to the first one if they are all
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