syscall: mark stdin, stdout and stderr as non-inheritable by child process

Fixes #2250.

R=golang-dev, hectorchu
CC=golang-dev, vincent.vanackere
https://golang.org/cl/5086050

src/pkg/syscall/syscall_windows.go

@@ -352,6 +352,7 @@ var (
 
 func getStdHandle(h int) (fd Handle) {
 	r, _ := GetStdHandle(h)
+	CloseOnExec(r)
 	return r
 }