Add fork of Go 1.15-dev's crypto/x509

Snapshotted from Go commit 619c7a48a38b28b521591b490fd14ccb7ea5e821
(https://go-review.googlesource.com/c/go/+/229762,
"crypto/x509: add x509omitbundledroots build tag to not embed roots")

With 975c01342a25899962969833d8b2873dc8856a4f
(https://go-review.googlesource.com/c/go/+/220721) removed, because it
depends on other stuff in Go std that doesn't yet exist in a Go
release.

Also, add a subset fork of Go's internal/testenv, for use by x509's tests.

tempfork/x509/root_unix.go

@@ -0,0 +1,92 @@
+// 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.
+
+// +build aix dragonfly freebsd js,wasm linux netbsd openbsd solaris
+
+package x509
+
+import (
+	"io/ioutil"
+	"os"
+	"strings"
+)
+
+// Possible directories with certificate files; stop after successfully
+// reading at least one file from a directory.
+var certDirectories = []string{
+	"/etc/ssl/certs",               // SLES10/SLES11, https://golang.org/issue/12139
+	"/system/etc/security/cacerts", // Android
+	"/usr/local/share/certs",       // FreeBSD
+	"/etc/pki/tls/certs",           // Fedora/RHEL
+	"/etc/openssl/certs",           // NetBSD
+	"/var/ssl/certs",               // AIX
+}
+
+const (
+	// certFileEnv is the environment variable which identifies where to locate
+	// the SSL certificate file. If set this overrides the system default.
+	certFileEnv = "SSL_CERT_FILE"
+
+	// certDirEnv is the environment variable which identifies which directory
+	// to check for SSL certificate files. If set this overrides the system default.
+	// It is a colon separated list of directories.
+	// See https://www.openssl.org/docs/man1.0.2/man1/c_rehash.html.
+	certDirEnv = "SSL_CERT_DIR"
+)
+
+func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate, err error) {
+	return nil, nil
+}
+
+func loadSystemRoots() (*CertPool, error) {
+	roots := NewCertPool()
+
+	files := certFiles
+	if f := os.Getenv(certFileEnv); f != "" {
+		files = []string{f}
+	}
+
+	var firstErr error
+	for _, file := range files {
+		data, err := ioutil.ReadFile(file)
+		if err == nil {
+			roots.AppendCertsFromPEM(data)
+			break
+		}
+		if firstErr == nil && !os.IsNotExist(err) {
+			firstErr = err
+		}
+	}
+
+	dirs := certDirectories
+	if d := os.Getenv(certDirEnv); d != "" {
+		// OpenSSL and BoringSSL both use ":" as the SSL_CERT_DIR separator.
+		// See:
+		//  * https://golang.org/issue/35325
+		//  * https://www.openssl.org/docs/man1.0.2/man1/c_rehash.html
+		dirs = strings.Split(d, ":")
+	}
+
+	for _, directory := range dirs {
+		fis, err := ioutil.ReadDir(directory)
+		if err != nil {
+			if firstErr == nil && !os.IsNotExist(err) {
+				firstErr = err
+			}
+			continue
+		}
+		for _, fi := range fis {
+			data, err := ioutil.ReadFile(directory + "/" + fi.Name())
+			if err == nil {
+				roots.AppendCertsFromPEM(data)
+			}
+		}
+	}
+
+	if len(roots.certs) > 0 || firstErr == nil {
+		return roots, nil
+	}
+
+	return nil, firstErr
+}