Merge of r4614, r4624-r4629, r4631: proxy recursive changes.

Merge of r4614, r4624-r4629, r4631: proxy recursive changes.

*) Added IPv6 and UNIX-domain socket support in "debug_connection"
   directive.

*) New function ngx_http_get_forwarded_addr() to look up real client
   address.

   On input it takes an original address, string in the X-Forwarded-For format
   and its length, list of trusted proxies, and a flag indicating to perform
   the recursive search.  On output it returns NGX_OK and the "deepest" valid
   address in a chain, or NGX_DECLINED.  It supports AF_INET and AF_INET6.
   Additionally, original address and/or proxy may be specified as AF_UNIX.

*) Realip: chains of trusted proxies and IPv6 support.

   The module now supports recursive search of client address through
   the chain of trusted proxies, controlled by the "real_ip_recursive"
   directive (closes #2).  It also gets full IPv6 support (closes #44)
   and canonical value of the $client_addr variable on address change.

   Example:

       real_ip_header X-Forwarded-For;
       set_real_ip_from 127.0.0.0/8;
       set_real_ip_from ::1;
       set_real_ip_from unix:;
       real_ip_recursive on;

*) Geo: chains of trusted proxies and partial IPv6 support.

   The module now supports recursive search of client address through
   the chain of trusted proxies, controlled by the "proxy_recursive"
   directive in the "geo" block.  It also gets partial IPv6 support:
   now proxies may be specified with IPv6 addresses.

   Example:

       geo $test {
           ...
           proxy 127.0.0.1;
           proxy ::1;
           proxy_recursive;
       }

   There's also a slight change in behavior.  When original client
   address (as specified by the "geo" directive) is one of the
   trusted proxies, and the value of the X-Forwarded-For request
   header cannot not be parsed as a valid address, an original client
   address will be used for lookup.  Previously, 255.255.255.255 was
   used in this case.

*) Geoip: trusted proxies support and partial IPv6 support.

   The module now supports recursive search of client address through the
   chain of trusted proxies (closes #100), in the same scope as the geo
   module.  Proxies are listed by the "geoip_proxy" directive, recursive
   search is enabled by the "geoip_proxy_recursive" directive.  IPv6 is
   partially supported: proxies may be specified with IPv6 addresses.

   Example:

        geoip_country .../GeoIP.dat;
        geoip_proxy 127.0.0.1;
        geoip_proxy ::1;
        geoip_proxy 10.0.0.0/8;
        geoip_proxy_recursive on;