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

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.