os: make sure Remove returns correct error on windows

R=golang-dev, bsiegert, rsc
CC=golang-dev
https://golang.org/cl/5493078

src/pkg/os/file_unix.go

@@ -210,6 +210,36 @@ func Truncate(name string, size int64) error {
 	return nil
 }
 
+// Remove removes the named file or directory.
+func Remove(name string) error {
+	// System call interface forces us to know
+	// whether name is a file or directory.
+	// Try both: it is cheaper on average than
+	// doing a Stat plus the right one.
+	e := syscall.Unlink(name)
+	if e == nil {
+		return nil
+	}
+	e1 := syscall.Rmdir(name)
+	if e1 == nil {
+		return nil
+	}
+
+	// Both failed: figure out which error to return.
+	// OS X and Linux differ on whether unlink(dir)
+	// returns EISDIR, so can't use that.  However,
+	// both agree that rmdir(file) returns ENOTDIR,
+	// so we can use that to decide which error is real.
+	// Rmdir might also return ENOTDIR if given a bad
+	// file path, like /etc/passwd/foo, but in that case,
+	// both errors will be ENOTDIR, so it's okay to
+	// use the error from unlink.
+	if e1 != syscall.ENOTDIR {
+		e = e1
+	}
+	return &PathError{"remove", name, e}
+}
+
 // basename removes trailing slashes and the leading directory name from path name
 func basename(name string) string {
 	i := len(name) - 1