all: replace io/ioutil with io and os package

For golang/go#45557

Change-Id: I447530cc66896aef7a8d528ccb8d095b80e3cf47
GitHub-Last-Rev: 5f385ff46487ac318bd1147cdbbd26bb0ffd0426
GitHub-Pull-Request: golang/crypto#230
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/430797
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

ssh/session.go

@@ -13,7 +13,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"sync"
 )
 
@@ -124,7 +123,7 @@ type Session struct {
 	// output and error.
 	//
 	// If either is nil, Run connects the corresponding file
-	// descriptor to an instance of ioutil.Discard. There is a
+	// descriptor to an instance of io.Discard. There is a
 	// fixed amount of buffering that is shared for the two streams.
 	// If either blocks it may eventually cause the remote
 	// command to block.
@@ -506,7 +505,7 @@ func (s *Session) stdout() {
 		return
 	}
 	if s.Stdout == nil {
-		s.Stdout = ioutil.Discard
+		s.Stdout = io.Discard
 	}
 	s.copyFuncs = append(s.copyFuncs, func() error {
 		_, err := io.Copy(s.Stdout, s.ch)
@@ -519,7 +518,7 @@ func (s *Session) stderr() {
 		return
 	}
 	if s.Stderr == nil {
-		s.Stderr = ioutil.Discard
+		s.Stderr = io.Discard
 	}
 	s.copyFuncs = append(s.copyFuncs, func() error {
 		_, err := io.Copy(s.Stderr, s.ch.Stderr())