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

windows/syscall_test.go

@@ -0,0 +1,30 @@
+// Copyright 2013 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 syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+func testSetGetenv(t *testing.T, key, value string) {
+	err := syscall.Setenv(key, value)
+	if err != nil {
+		t.Fatalf("Setenv failed to set %q: %v", value, err)
+	}
+	newvalue, found := syscall.Getenv(key)
+	if !found {
+		t.Fatalf("Getenv failed to find %v variable (want value %q)", key, value)
+	}
+	if newvalue != value {
+		t.Fatalf("Getenv(%v) = %q; want %q", key, newvalue, value)
+	}
+}
+
+func TestEnv(t *testing.T) {
+	testSetGetenv(t, "TESTENV", "AVALUE")
+	// make sure TESTENV gets set to "", not deleted
+	testSetGetenv(t, "TESTENV", "")
+}