[MEDIUM] http: add support for proxy authentication

We're already able to know if a request is a proxy request or a
normal one, and we have an option "http-use-proxy-header" which states
that proxy headers must be checked. So let's switch to use the proxy
authentication headers and responses when this option is set and we're
facing a proxy request. That allows haproxy to enforce auth in front
of a proxy.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 2ef3639..48c90c7 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2691,6 +2691,11 @@
   that this option can only be specified in a frontend and will affect the
   request along its whole life.
 
+  Also, when this option is set, a request which requires authentication will
+  automatically switch to use proxy authentication headers if it is itself a
+  proxied request. That makes it possible to check or enforce authentication in
+  front of an existing proxy.
+
   This option should normally never be used, except in front of a proxy.
 
   See also : "option httpclose", "option forceclose" and "option