libbnetservices: BUrlResult is no longer a BArchivable

The switch to make BUrlResult serializable was debuted in
f9e1854f19 with the rationale is that
BHttpRequest auto-redirection might cause the headers to become
obsolete by the time a client process the BMessage received from
BUrlProtocolDispatchingListener.

With the change to BHttpRequest to not notify listeners when
auto-redirection is enabled, this is no longer the case and the
serialization code can go away now. This simplifies BUrlResult and its
subclasses, and gain us some performance for clients using
BUrlProtocolDispatchingListener as the result object no longer has to be
serialized.

This also change the ABI of BUrlProtocolListener::HeadersReceived to no
longer passing a BUrlResult.

Additionally, BUrlResult and BHttpResult now express the size of the content
as an off_t, thus allowing results larger than 4 GB.

Change-Id: I9dd29a8b26fdd9aa8e5bbad8d1728084f136312d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3082
Reviewed-by: Adrien Destugues <[email protected]>

docs/user/netservices/UrlProtocolListener.dox

@@ -103,15 +103,13 @@
 */
 
 /*!
-	\fn virtual void BUrlProtocolListener::HeadersReceived(BUrlRequest* caller,
-		const BUrlResult& result);
+	\fn virtual void BUrlProtocolListener::HeadersReceived(BUrlRequest* caller)
 	\brief Called when all of the server response metadata (such as headers)
 		have been read and parsed.
 
 	\b Frequency: Once
 
 	\param caller The BUrlRequest that invoked this callback.
-	\param result The BUrlResult associated with the request.
 */
 
 /*!