Add uci.Engine.Getpid()

This change adds a Getpid() method receiver to uci.Engine. This
enables callers to interact with the engine process directly. For
example callers may renice the process to allow the engine to run
at a lower priority. In practice this is helpful in improving system
responsiveness when running the engine with high thread counts and/or
larger hash sizes.

uci/engine.go

@@ -55,10 +55,18 @@ func New(path string, opts ...func(e *Engine)) (*Engine, error) {
 	for _, opt := range opts {
 		opt(e)
 	}
-	go e.cmd.Run()
+	err = e.cmd.Start()
+	if err != nil {
+		return nil, fmt.Errorf("uci: failed to start executable %s: %w", path, err)
+	}
+	go e.cmd.Wait()
 	return e, nil
 }
 
+func (e *Engine) Getpid() int {
+	return e.cmd.Process.Pid
+}
+
 // ID returns the id values returned from the most recent CmdUCI invocation.  It includes
 // key value data such as the following:
 // id name Stockfish 12