Improve HTTP authentication support.

The authentication state is stored (in a hash map, using the domain+path
as a key) in the UrlContext class. It can then be reused for multiple
requests to the same place. We also lookup stored authentications for
parent directories and stop at the first we find.

Authentication state is not stored on disk (unlike cookies), and there
can only be one for each domain+path.

headers/os/net/HttpRequest.h

@@ -9,7 +9,6 @@
 #include <deque>
 
 
-#include <HttpAuthentication.h>
 #include <HttpForm.h>
 #include <HttpHeaders.h>
 #include <HttpResult.h>
@@ -93,7 +92,6 @@ private:
 			BNetBuffer			fInputBuffer;
 
 			BHttpHeaders		fHeaders;
-			BHttpAuthentication	fAuthentication;
 
 	// Request status