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_sendbuf.c

@@ -148,6 +148,7 @@ done:
 
     sb->buf = b;
     sb->size = total;
+    sb->niov = n;
 
     return n;
 }
@@ -390,6 +391,14 @@ nxt_sendbuf_completion(nxt_task_t *task, nxt_work_queue_t *wq, nxt_buf_t *b,
                     break;
                 }
 
+                if (nxt_buf_is_port_mmap(b)) {
+                    /*
+                     * buffer has been sent to other side which is now
+                     * responsible for shared memory bucket release
+                     */
+                    b->is_port_mmap_sent = 1;
+                }
+
                 if (sent < size) {
 
                     if (nxt_buf_is_mem(b)) {