crypto/tls: fetch root certificates using Mac OS API

Fixes #1009.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/5262041

src/pkg/crypto/tls/root_unix.go

@@ -0,0 +1,27 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"crypto/x509"
+	"io/ioutil"
+)
+
+// Possible certificate files; stop after finding one.
+var certFiles = []string{
+	"/etc/ssl/certs/ca-certificates.crt", // Linux etc
+}
+
+func initDefaultRoots() {
+	roots := x509.NewCertPool()
+	for _, file := range certFiles {
+		data, err := ioutil.ReadFile(file)
+		if err == nil {
+			roots.AppendCertsFromPEM(data)
+			break
+		}
+	}
+	varDefaultRoots = roots
+}