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/main.cpp

@@ -77,6 +77,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "subgame.h"
 #include "quicktune.h"
 #include "serverlist.h"
+#include "httpfetch.h"
 #include "guiEngine.h"
 #include "mapsector.h"
 
@@ -1001,6 +1002,9 @@ int main(int argc, char *argv[])
 	assert(res == CURLE_OK);
 #endif
 
+	// Initialize HTTP fetcher
+	httpfetch_init(g_settings->getS32("curl_parallel_limit"));
+
 	/*
 		Run unit tests
 	*/
@@ -1858,6 +1862,9 @@ int main(int argc, char *argv[])
 		}
 	}
 
+	// Stop httpfetch thread (if started)
+	httpfetch_cleanup();
+
 	END_DEBUG_EXCEPTION_HANDLER(errorstream)
 
 	debugstreams_deinit();