os: add FileMode.IsRegular

API change.

R=golang-dev, r, iant, rsc
CC=golang-dev
https://golang.org/cl/6844048

src/pkg/os/types.go

@@ -88,6 +88,12 @@ func (m FileMode) IsDir() bool {
 	return m&ModeDir != 0
 }
 
+// IsRegular reports whether m describes a regular file.
+// That is, it tests that no mode type bits are set.
+func (m FileMode) IsRegular() bool {
+	return m&ModeType == 0
+}
+
 // Perm returns the Unix permission bits in m.
 func (m FileMode) Perm() FileMode {
 	return m & ModePerm