os: new FileInfo, FileMode types + update tree

R=golang-dev, r, r, gri, bradfitz, iant, iant, nigeltao, n13m3y3r
CC=golang-dev
https://golang.org/cl/5416060
diff --git a/src/pkg/os/exec/lp_unix.go b/src/pkg/os/exec/lp_unix.go
index d234641..9665ea8 100644
--- a/src/pkg/os/exec/lp_unix.go
+++ b/src/pkg/os/exec/lp_unix.go
@@ -20,7 +20,7 @@
 	if err != nil {
 		return err
 	}
-	if d.IsRegular() && d.Permission()&0111 != 0 {
+	if m := d.Mode(); !m.IsDir() && m&0111 != 0 {
 		return nil
 	}
 	return os.EPERM