Preserving the app port write socket.

The socket is required for intercontextual communication in multithreaded apps.

src/nxt_application.c

@@ -723,7 +723,7 @@ nxt_unit_default_init(nxt_task_t *task, nxt_unit_init_t *init)
     init->read_port.id.pid = my_port->pid;
     init->read_port.id.id = my_port->id;
     init->read_port.in_fd = my_port->pair[0];
-    init->read_port.out_fd = -1;
+    init->read_port.out_fd = my_port->pair[1];
 
     init->log_fd = 2;