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

plan9/env_plan9.go

@@ -0,0 +1,142 @@
+// 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.
+
+// Plan 9 environment variables.
+
+package syscall
+
+import (
+	"errors"
+	"sync"
+)
+
+var (
+	// envOnce guards copyenv, which populates env.
+	envOnce sync.Once
+
+	// envLock guards env and envs.
+	envLock sync.RWMutex
+
+	// env maps from an environment variable to its value.
+	env = make(map[string]string)
+
+	// envs contains elements of env in the form "key=value".
+	envs []string
+
+	errZeroLengthKey = errors.New("zero length key")
+	errShortWrite    = errors.New("i/o count too small")
+)
+
+func readenv(key string) (string, error) {
+	fd, err := Open("/env/"+key, O_RDONLY)
+	if err != nil {
+		return "", err
+	}
+	defer Close(fd)
+	l, _ := Seek(fd, 0, 2)
+	Seek(fd, 0, 0)
+	buf := make([]byte, l)
+	n, err := Read(fd, buf)
+	if err != nil {
+		return "", err
+	}
+	if n > 0 && buf[n-1] == 0 {
+		buf = buf[:n-1]
+	}
+	return string(buf), nil
+}
+
+func writeenv(key, value string) error {
+	fd, err := Create("/env/"+key, O_RDWR, 0666)
+	if err != nil {
+		return err
+	}
+	defer Close(fd)
+	b := []byte(value)
+	n, err := Write(fd, b)
+	if err != nil {
+		return err
+	}
+	if n != len(b) {
+		return errShortWrite
+	}
+	return nil
+}
+
+func copyenv() {
+	fd, err := Open("/env", O_RDONLY)
+	if err != nil {
+		return
+	}
+	defer Close(fd)
+	files, err := readdirnames(fd)
+	if err != nil {
+		return
+	}
+	envs = make([]string, len(files))
+	i := 0
+	for _, key := range files {
+		v, err := readenv(key)
+		if err != nil {
+			continue
+		}
+		env[key] = v
+		envs[i] = key + "=" + v
+		i++
+	}
+}
+
+func Getenv(key string) (value string, found bool) {
+	if len(key) == 0 {
+		return "", false
+	}
+
+	envLock.RLock()
+	defer envLock.RUnlock()
+
+	if v, ok := env[key]; ok {
+		return v, true
+	}
+	v, err := readenv(key)
+	if err != nil {
+		return "", false
+	}
+	env[key] = v
+	envs = append(envs, key+"="+v)
+	return v, true
+}
+
+func Setenv(key, value string) error {
+	if len(key) == 0 {
+		return errZeroLengthKey
+	}
+
+	envLock.Lock()
+	defer envLock.Unlock()
+
+	err := writeenv(key, value)
+	if err != nil {
+		return err
+	}
+	env[key] = value
+	envs = append(envs, key+"="+value)
+	return nil
+}
+
+func Clearenv() {
+	envLock.Lock()
+	defer envLock.Unlock()
+
+	env = make(map[string]string)
+	envs = []string{}
+	RawSyscall(SYS_RFORK, RFCENVG, 0, 0)
+}
+
+func Environ() []string {
+	envLock.RLock()
+	defer envLock.RUnlock()
+
+	envOnce.Do(copyenv)
+	return append([]string(nil), envs...)
+}