Add file RPC dup call

src/main.rs

@@ -56,6 +56,10 @@ impl file_rpc::Server for FileServ {
     fn size(&self, _fd: u64) -> Option<u64> {
         None
     }
+    
+    fn dup(&self, fd: u64) -> Option<u64> {
+        Some(fd)
+    }
 }
 
 static SCANDODE_DECODER: Mutex<ScancodeSet2> = Mutex::new(ScancodeSet2::new());