fix golint warnings/errors on pkg/system and pkg/stdcopy

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

system/umask.go

@@ -6,6 +6,8 @@ import (
 	"syscall"
 )
 
+// Umask sets current process's file mode creation mask to newmask
+// and return oldmask.
 func Umask(newmask int) (oldmask int, err error) {
 	return syscall.Umask(newmask), nil
 }