Add a fail_cb to evhttp_request for connection failures

The callback will be executed if a failure (timeout, read error) is detected
on an incoming connection.

This can be used to detect a client closing the connection while engaged in a
chunked reply and actively pushing chunks to the client. Without this
notification, it is impossible to know if/when a connection gets closed and
react appropriately (ie. stop pushing chunks) resulting in a segfault.

src/evhttp/evhttp.h

@@ -229,6 +229,9 @@ struct {
 	void (*cb)(struct evhttp_request *, void *);
 	void *cb_arg;
 
+	void (*fail_cb)(struct evhttp_request *, void *);
+	void *fail_cb_arg;
+
 	/*
 	 * Chunked data callback - call for each completed chunk if
 	 * specified.  If not specified, all the data is delivered via

src/evhttp/http.c

@@ -650,7 +650,11 @@ evhttp_connection_fail(struct evhttp_connection *evcon,
 		 * reply before the connection can be freed.
 		 */
 		if (evhttp_connection_incoming_fail(req, error) == -1)
-			evhttp_connection_free(evcon);
+		  {
+		    if (req->fail_cb)
+		      req->fail_cb(req, req->fail_cb_arg);
+		    evhttp_connection_free(evcon);
+		  }
 		return;
 	}