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/syscall.go

@@ -0,0 +1,81 @@
+// Copyright 2009 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 contains an interface to the low-level operating system
+// primitives.  The details vary depending on the underlying system, and
+// by default, godoc will display the syscall documentation for the current
+// system.  If you want godoc to display syscall documentation for another
+// system, set $GOOS and $GOARCH to the desired system.  For example, if
+// you want to view documentation for freebsd/arm on linux/amd64, set $GOOS
+// to freebsd and $GOARCH to arm.
+// The primary use of syscall is inside other packages that provide a more
+// portable interface to the system, such as "os", "time" and "net".  Use
+// those packages rather than this one if you can.
+// For details of the functions and data types in this package consult
+// the manuals for the appropriate operating system.
+// These calls return err == nil to indicate success; otherwise
+// err is an operating system error describing the failure.
+// On most systems, that error has type syscall.Errno.
+package syscall
+
+// StringByteSlice is deprecated. Use ByteSliceFromString instead.
+// If s contains a NUL byte this function panics instead of
+// returning an error.
+func StringByteSlice(s string) []byte {
+	a, err := ByteSliceFromString(s)
+	if err != nil {
+		panic("syscall: string with NUL passed to StringByteSlice")
+	}
+	return a
+}
+
+// ByteSliceFromString returns a NUL-terminated slice of bytes
+// containing the text of s. If s contains a NUL byte at any
+// location, it returns (nil, EINVAL).
+func ByteSliceFromString(s string) ([]byte, error) {
+	for i := 0; i < len(s); i++ {
+		if s[i] == 0 {
+			return nil, EINVAL
+		}
+	}
+	a := make([]byte, len(s)+1)
+	copy(a, s)
+	return a, nil
+}
+
+// StringBytePtr is deprecated. Use BytePtrFromString instead.
+// If s contains a NUL byte this function panics instead of
+// returning an error.
+func StringBytePtr(s string) *byte { return &StringByteSlice(s)[0] }
+
+// BytePtrFromString returns a pointer to a NUL-terminated array of
+// bytes containing the text of s. If s contains a NUL byte at any
+// location, it returns (nil, EINVAL).
+func BytePtrFromString(s string) (*byte, error) {
+	a, err := ByteSliceFromString(s)
+	if err != nil {
+		return nil, err
+	}
+	return &a[0], nil
+}
+
+// Single-word zero for use when we need a valid pointer to 0 bytes.
+// See mksyscall.pl.
+var _zero uintptr
+
+func (ts *Timespec) Unix() (sec int64, nsec int64) {
+	return int64(ts.Sec), int64(ts.Nsec)
+}
+
+func (tv *Timeval) Unix() (sec int64, nsec int64) {
+	return int64(tv.Sec), int64(tv.Usec) * 1000
+}
+
+func (ts *Timespec) Nano() int64 {
+	return int64(ts.Sec)*1e9 + int64(ts.Nsec)
+}
+
+func (tv *Timeval) Nano() int64 {
+	return int64(tv.Sec)*1e9 + int64(tv.Usec)*1000
+}