Implement HTTP chunked streaming

Transcoding is not supported yet.

In the current state of affairs, we will crash if the client closes the
connection before the tranfser is done. This is a limitation of evhttp,
it is not possible to detect this condition. This will need to be fixed
in evhttp.

src/httpd.h

@@ -2,6 +2,13 @@
 #ifndef __HTTPD_H__
 #define __HTTPD_H__
 
+#include <event.h>
+#include <evhttp.h>
+
+
+void
+httpd_stream_file(struct evhttp_request *req, int id);
+
 int
 httpd_init(void);