MINOR: http-ana: Use proxy's error replies to emit 401/407 responses

There is no reason to not use proxy's error replies to emit 401/407
responses. The function http_reply_40x_unauthorized(), responsible to emit those
responses, is not really complex. It only adds a
WWW-Authenticate/Proxy-Authenticate header to a generic message.

So now, error replies can be defined for 401 and 407 status codes, using
errorfile or http-error directives. When an http-request auth rule is evaluated,
the corresponding error reply is used. For 401 responses, all occurrences of the
WWW-Authenticate header are removed and replaced by a new one with a basic
authentication challenge for the configured realm. For 407 responses, the same
is done on the Proxy-Authenticate header. If the error reply must not be
altered, "http-request return" rule must be used instead.
diff --git a/include/common/http.h b/include/common/http.h
index 6f083d4..d31f5e9 100644
--- a/include/common/http.h
+++ b/include/common/http.h
@@ -82,9 +82,11 @@
 enum {
 	HTTP_ERR_200 = 0,
 	HTTP_ERR_400,
+	HTTP_ERR_401,
 	HTTP_ERR_403,
 	HTTP_ERR_404,
 	HTTP_ERR_405,
+	HTTP_ERR_407,
 	HTTP_ERR_408,
 	HTTP_ERR_410,
 	HTTP_ERR_421,