log: add SetOutput method on Logger

Fixes #9629

Change-Id: I66091003b97742ca6d857fe51d609833ab727216
Reviewed-on: https://go-review.googlesource.com/3023Reviewed-by: Rob Pike <r@golang.org>

src/log/log.go

@@ -60,6 +60,13 @@ func New(out io.Writer, prefix string, flag int) *Logger {
 	return &Logger{out: out, prefix: prefix, flag: flag}
 }
 
+// SetOutput sets the output destination for the logger.
+func (l *Logger) SetOutput(w io.Writer) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	l.out = w
+}
+
 var std = New(os.Stderr, "", LstdFlags)
 
 // Cheap integer to fixed-width decimal ASCII.  Give a negative width to avoid zero-padding.