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/lsf_linux.go

@@ -0,0 +1,78 @@
+// 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.
+
+// Linux socket filter
+
+package syscall
+
+import (
+	"unsafe"
+)
+
+func LsfStmt(code, k int) *SockFilter {
+	return &SockFilter{Code: uint16(code), K: uint32(k)}
+}
+
+func LsfJump(code, k, jt, jf int) *SockFilter {
+	return &SockFilter{Code: uint16(code), Jt: uint8(jt), Jf: uint8(jf), K: uint32(k)}
+}
+
+func LsfSocket(ifindex, proto int) (int, error) {
+	var lsall SockaddrLinklayer
+	s, e := Socket(AF_PACKET, SOCK_RAW, proto)
+	if e != nil {
+		return 0, e
+	}
+	p := (*[2]byte)(unsafe.Pointer(&lsall.Protocol))
+	p[0] = byte(proto >> 8)
+	p[1] = byte(proto)
+	lsall.Ifindex = ifindex
+	e = Bind(s, &lsall)
+	if e != nil {
+		Close(s)
+		return 0, e
+	}
+	return s, nil
+}
+
+type iflags struct {
+	name  [IFNAMSIZ]byte
+	flags uint16
+}
+
+func SetLsfPromisc(name string, m bool) error {
+	s, e := Socket(AF_INET, SOCK_DGRAM, 0)
+	if e != nil {
+		return e
+	}
+	defer Close(s)
+	var ifl iflags
+	copy(ifl.name[:], []byte(name))
+	_, _, ep := Syscall(SYS_IOCTL, uintptr(s), SIOCGIFFLAGS, uintptr(unsafe.Pointer(&ifl)))
+	if ep != 0 {
+		return Errno(ep)
+	}
+	if m {
+		ifl.flags |= uint16(IFF_PROMISC)
+	} else {
+		ifl.flags &= ^uint16(IFF_PROMISC)
+	}
+	_, _, ep = Syscall(SYS_IOCTL, uintptr(s), SIOCSIFFLAGS, uintptr(unsafe.Pointer(&ifl)))
+	if ep != 0 {
+		return Errno(ep)
+	}
+	return nil
+}
+
+func AttachLsf(fd int, i []SockFilter) error {
+	var p SockFprog
+	p.Len = uint16(len(i))
+	p.Filter = (*SockFilter)(unsafe.Pointer(&i[0]))
+	return setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, unsafe.Pointer(&p), unsafe.Sizeof(p))
+}
+
+func DetachLsf(fd int) error {
+	var dummy int
+	return setsockopt(fd, SOL_SOCKET, SO_DETACH_FILTER, unsafe.Pointer(&dummy), unsafe.Sizeof(dummy))
+}