Implement httpfetch module and initialize it from main()

Add curl_parallel_limit setting that will replace media_fetch_threads in
a later commit.

Fix a typo in MutexedQueue::pop_back() that made it impossible to compile
code that used this function. (Noticed this while implementing httpfetch.)

src/httpfetch.h

@@ -0,0 +1,126 @@
+/*
+Minetest
+Copyright (C) 2013 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#ifndef HTTPFETCH_HEADER
+#define HTTPFETCH_HEADER
+
+#include <string>
+#include <vector>
+#include "config.h"
+
+// Can be used in place of "caller" in asynchronous transfers to discard result
+// (used as default value of "caller")
+#define HTTPFETCH_DISCARD 0
+
+struct HTTPFetchRequest
+{
+	std::string url;
+
+	// Identifies the caller (for asynchronous requests)
+	// Ignored by httpfetch_sync
+	unsigned long caller;
+
+	// Some number that identifies the request
+	// (when the same caller issues multiple httpfetch_async calls)
+	unsigned long request_id;
+
+	// Timeout for the whole transfer, in milliseconds
+	long timeout;
+
+	// Timeout for the connection phase, in milliseconds
+	long connect_timeout;
+
+	// POST data (should be application/x-www-form-urlencoded
+	// unless a Content-Type header is specified in extra_headers)
+	// If this is empty a GET request is done instead.
+	std::string post_fields;
+
+	// If not empty, should contain entries such as "Accept: text/html"
+	std::vector<std::string> extra_headers;
+
+	HTTPFetchRequest()
+	{
+		url = "";
+		caller = HTTPFETCH_DISCARD;
+		request_id = 0;
+		timeout = 0;
+		connect_timeout = 0;
+	}
+};
+
+struct HTTPFetchResult
+{
+	bool succeeded;
+	bool timeout;
+	long response_code;
+	std::string data;
+	// The caller and request_id from the corresponding HTTPFetchRequest.
+	unsigned long caller;
+	unsigned long request_id;
+
+	HTTPFetchResult()
+	{
+		succeeded = false;
+		timeout = false;
+		response_code = 0;
+		data = "";
+		caller = HTTPFETCH_DISCARD;
+		request_id = 0;
+	}
+
+	HTTPFetchResult(const HTTPFetchRequest &fetchrequest)
+	{
+		succeeded = false;
+		timeout = false;
+		response_code = 0;
+		data = "";
+		caller = fetchrequest.caller;
+		request_id = fetchrequest.request_id;
+	}
+};
+
+// Initializes the httpfetch module
+void httpfetch_init(int parallel_limit);
+
+// Stops the httpfetch thread and cleans up resources
+void httpfetch_cleanup();
+
+// Starts an asynchronous HTTP fetch request
+void httpfetch_async(const HTTPFetchRequest &fetchrequest);
+
+// If any fetch for the given caller ID is complete, removes it from the
+// result queue, sets fetchresult and returns true. Otherwise returns false.
+bool httpfetch_async_get(unsigned long caller, HTTPFetchResult &fetchresult);
+
+// Allocates a caller ID for httpfetch_async
+// Not required if you want to set caller = HTTPFETCH_DISCARD
+unsigned long httpfetch_caller_alloc();
+
+// Frees a caller ID allocated with httpfetch_caller_alloc
+// Note: This can be expensive, because the httpfetch thread is told
+// to stop any ongoing fetches for the given caller.
+void httpfetch_caller_free(unsigned long caller);
+
+// Performs a synchronous HTTP request. This blocks and therefore should
+// only be used from background threads.
+void httpfetch_sync(const HTTPFetchRequest &fetchrequest,
+		HTTPFetchResult &fetchresult);
+
+
+#endif // !HTTPFETCH_HEADER