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/env_windows.go

@@ -0,0 +1,82 @@
+// Copyright 2010 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.
+
+// Windows environment variables.
+
+package syscall
+
+import (
+	"unicode/utf16"
+	"unsafe"
+)
+
+func Getenv(key string) (value string, found bool) {
+	keyp, err := UTF16PtrFromString(key)
+	if err != nil {
+		return "", false
+	}
+	b := make([]uint16, 100)
+	n, e := GetEnvironmentVariable(keyp, &b[0], uint32(len(b)))
+	if n == 0 && e == ERROR_ENVVAR_NOT_FOUND {
+		return "", false
+	}
+	if n > uint32(len(b)) {
+		b = make([]uint16, n)
+		n, e = GetEnvironmentVariable(keyp, &b[0], uint32(len(b)))
+		if n > uint32(len(b)) {
+			n = 0
+		}
+	}
+	return string(utf16.Decode(b[0:n])), true
+}
+
+func Setenv(key, value string) error {
+	v, err := UTF16PtrFromString(value)
+	if err != nil {
+		return err
+	}
+	keyp, err := UTF16PtrFromString(key)
+	if err != nil {
+		return err
+	}
+	e := SetEnvironmentVariable(keyp, v)
+	if e != nil {
+		return e
+	}
+	return nil
+}
+
+func Clearenv() {
+	for _, s := range Environ() {
+		// Environment variables can begin with =
+		// so start looking for the separator = at j=1.
+		// http://blogs.msdn.com/b/oldnewthing/archive/2010/05/06/10008132.aspx
+		for j := 1; j < len(s); j++ {
+			if s[j] == '=' {
+				Setenv(s[0:j], "")
+				break
+			}
+		}
+	}
+}
+
+func Environ() []string {
+	s, e := GetEnvironmentStrings()
+	if e != nil {
+		return nil
+	}
+	defer FreeEnvironmentStrings(s)
+	r := make([]string, 0, 50) // Empty with room to grow.
+	for from, i, p := 0, 0, (*[1 << 24]uint16)(unsafe.Pointer(s)); true; i++ {
+		if p[i] == 0 {
+			// empty string marks the end
+			if i <= from {
+				break
+			}
+			r = append(r, string(utf16.Decode(p[from:i])))
+			from = i + 1
+		}
+	}
+	return r
+}