crypto/x509: add Plan 9 root certificate location

R=golang-dev
CC=golang-dev, rsc
https://golang.org/cl/6571056

src/pkg/crypto/x509/root_plan9.go

+// Copyright 2012 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.
+
+// +build plan9
+
+package x509
+
+import "io/ioutil"
+
+// Possible certificate files; stop after finding one.
+var certFiles = []string{
+	"/sys/lib/tls/ca.pem",
+}
+
+func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate, err error) {
+	return nil, nil
+}
+
+func initSystemRoots() {
+	roots := NewCertPool()
+	for _, file := range certFiles {
+		data, err := ioutil.ReadFile(file)
+		if err == nil {
+			roots.AppendCertsFromPEM(data)
+			break
+		}
+	}
+
+	systemRoots = roots
+}

src/pkg/crypto/x509/root_stub.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 darwin,!cgo
+// +build darwin,!cgo
 
 package x509