os: add ModeType constant to mask file type bits

This covers the lack of IsRegular comfortably:

    if stat.Mode()&os.ModeType == 0 { ... }

R=golang-dev, r, rsc, r, gustavo
CC=golang-dev
https://golang.org/cl/5440075

src/pkg/os/types.go

@@ -44,6 +44,9 @@ const (
 	ModeSetuid                                    // u: setuid
 	ModeSetgid                                    // g: setgid
 
+	// Mask for the type bits. For regular files, none will be set.
+	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice
+
 	ModePerm FileMode = 0777 // permission bits
 )