Add ResponderHeadersOnly.

ResponderHeadersOnly is a base class for responders that use
HTTPClient::head or HTTPClient::getHeaderOnly. It already
has a needsHeaders() that return true and only allows for
completedHeaders to be overridden.

I removed the CURLOPT_HEADER option for these cases, because
that only causes the headers to be send to the writeCallback
as if they are part of the body, in addition to the headerCallback;
That gave raise to some confusion for the existing code (ie,
unexpected errors when trying to decode the body as LLSD and
duplicated 'low speed' information for the Timeout policy code.

indra/llmessage/llurlrequest.cpp

@@ -212,7 +212,6 @@ bool LLURLRequest::configure(AICurlEasyRequest_wat const& curlEasyRequest_w)
 		switch(mAction)
 		{
 		case HTTP_HEAD:
-			curlEasyRequest_w->setopt(CURLOPT_HEADER, 1);
 			curlEasyRequest_w->setopt(CURLOPT_NOBODY, 1);
 			curlEasyRequest_w->setopt(CURLOPT_FOLLOWLOCATION, 1);
 			rv = true;