Using shared memory to send data via nxt_port.

Usage:
    b = nxt_port_mmap_get_buf(task, port, size);
    b->mem.free = nxt_cpymem(b->mem.free, data, size);
    nxt_port_socket_write(task, port, NXT_PORT_MSG_DATA, -1, 0, b);

src/nxt_worker_process.c

@@ -25,6 +25,7 @@ nxt_port_handler_t  nxt_worker_process_port_handlers[] = {
     nxt_worker_process_quit_handler,
     nxt_port_new_port_handler,
     nxt_port_change_log_file_handler,
+    nxt_port_mmap_handler,
     nxt_port_data_handler,
 };