Add HTTP proxy support.

* Move default context management to BUrlRequest since some code
(including the testsuite) bypass the BUrlProtocolRoster.
* Introduce proxy host and port in BUrlContext
* Have BHttpRequest use the proxy when making requests

headers/os/net/NetworkRequest.h

@@ -26,7 +26,7 @@ public:
 	virtual	status_t			Stop();
 
 protected:
-			bool 				_ResolveHostName(uint16_t port);
+			bool 				_ResolveHostName(BString host, uint16_t port);
 
 			void				_ProtocolSetup();
 			status_t			_GetLine(BString& destString);

headers/os/net/UrlContext.h

@@ -2,6 +2,8 @@
  * Copyright 2010-2014 Haiku Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
+
+
 #ifndef _B_URL_CONTEXT_H_
 #define _B_URL_CONTEXT_H_
 
@@ -27,16 +29,24 @@ public:
 									const BNetworkCookieJar& cookieJar);
 			void				AddAuthentication(const BUrl& url,
 									const BHttpAuthentication& authentication);
-											
+			void				SetProxy(BString host, uint16 port);
+
 	// Context accessors
 			BNetworkCookieJar&	GetCookieJar();
 			BHttpAuthentication& GetAuthentication(const BUrl& url);
+			bool				UseProxy();
+			BString				GetProxyHost();
+			uint16				GetProxyPort();
 
 private:
 			BNetworkCookieJar	fCookieJar;
 			typedef BPrivate::SynchronizedHashMap<BPrivate::HashString,
 				BHttpAuthentication*> BHttpAuthenticationMap;
 			BHttpAuthenticationMap* fAuthenticationMap;
+
+			BString				fProxyHost;
+			uint16				fProxyPort;
 };
 
+
 #endif // _B_URL_CONTEXT_H_