go.sys: copy files from syscall package to go.sys/{plan9,windows,unix}

This CL copies to each package of go.sys the files from syscall it will need.
Different directories have different files, but these:
        mkall.sh
        str.go
        syscall.go
        mksyscall.pl
        race.go
        race0.go
        syscall_test.go
are copied to all three.
No changes yet, these are just copies. They are not ready to use yet:
package names are wrong, for starters. But this clean copy will make
it easier to follow the changes as the packages are enabled.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/126960043

unix/creds_test.go

@@ -0,0 +1,113 @@
+// 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 linux
+
+package syscall_test
+
+import (
+	"bytes"
+	"net"
+	"os"
+	"syscall"
+	"testing"
+)
+
+// TestSCMCredentials tests the sending and receiving of credentials
+// (PID, UID, GID) in an ancillary message between two UNIX
+// sockets. The SO_PASSCRED socket option is enabled on the sending
+// socket for this to work.
+func TestSCMCredentials(t *testing.T) {
+	fds, err := syscall.Socketpair(syscall.AF_LOCAL, syscall.SOCK_STREAM, 0)
+	if err != nil {
+		t.Fatalf("Socketpair: %v", err)
+	}
+	defer syscall.Close(fds[0])
+	defer syscall.Close(fds[1])
+
+	err = syscall.SetsockoptInt(fds[0], syscall.SOL_SOCKET, syscall.SO_PASSCRED, 1)
+	if err != nil {
+		t.Fatalf("SetsockoptInt: %v", err)
+	}
+
+	srvFile := os.NewFile(uintptr(fds[0]), "server")
+	defer srvFile.Close()
+	srv, err := net.FileConn(srvFile)
+	if err != nil {
+		t.Errorf("FileConn: %v", err)
+		return
+	}
+	defer srv.Close()
+
+	cliFile := os.NewFile(uintptr(fds[1]), "client")
+	defer cliFile.Close()
+	cli, err := net.FileConn(cliFile)
+	if err != nil {
+		t.Errorf("FileConn: %v", err)
+		return
+	}
+	defer cli.Close()
+
+	var ucred syscall.Ucred
+	if os.Getuid() != 0 {
+		ucred.Pid = int32(os.Getpid())
+		ucred.Uid = 0
+		ucred.Gid = 0
+		oob := syscall.UnixCredentials(&ucred)
+		_, _, err := cli.(*net.UnixConn).WriteMsgUnix(nil, oob, nil)
+		if err.(*net.OpError).Err != syscall.EPERM {
+			t.Fatalf("WriteMsgUnix failed with %v, want EPERM", err)
+		}
+	}
+
+	ucred.Pid = int32(os.Getpid())
+	ucred.Uid = uint32(os.Getuid())
+	ucred.Gid = uint32(os.Getgid())
+	oob := syscall.UnixCredentials(&ucred)
+
+	// this is going to send a dummy byte
+	n, oobn, err := cli.(*net.UnixConn).WriteMsgUnix(nil, oob, nil)
+	if err != nil {
+		t.Fatalf("WriteMsgUnix: %v", err)
+	}
+	if n != 0 {
+		t.Fatalf("WriteMsgUnix n = %d, want 0", n)
+	}
+	if oobn != len(oob) {
+		t.Fatalf("WriteMsgUnix oobn = %d, want %d", oobn, len(oob))
+	}
+
+	oob2 := make([]byte, 10*len(oob))
+	n, oobn2, flags, _, err := srv.(*net.UnixConn).ReadMsgUnix(nil, oob2)
+	if err != nil {
+		t.Fatalf("ReadMsgUnix: %v", err)
+	}
+	if flags != 0 {
+		t.Fatalf("ReadMsgUnix flags = 0x%x, want 0", flags)
+	}
+	if n != 1 {
+		t.Fatalf("ReadMsgUnix n = %d, want 1 (dummy byte)", n)
+	}
+	if oobn2 != oobn {
+		// without SO_PASSCRED set on the socket, ReadMsgUnix will
+		// return zero oob bytes
+		t.Fatalf("ReadMsgUnix oobn = %d, want %d", oobn2, oobn)
+	}
+	oob2 = oob2[:oobn2]
+	if !bytes.Equal(oob, oob2) {
+		t.Fatal("ReadMsgUnix oob bytes don't match")
+	}
+
+	scm, err := syscall.ParseSocketControlMessage(oob2)
+	if err != nil {
+		t.Fatalf("ParseSocketControlMessage: %v", err)
+	}
+	newUcred, err := syscall.ParseUnixCredentials(&scm[0])
+	if err != nil {
+		t.Fatalf("ParseUnixCredentials: %v", err)
+	}
+	if *newUcred != ucred {
+		t.Fatalf("ParseUnixCredentials = %+v, want %+v", newUcred, ucred)
+	}
+}