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/route_openbsd.go

@@ -0,0 +1,35 @@
+// 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.
+
+// Routing sockets and messages for OpenBSD
+
+package syscall
+
+import "unsafe"
+
+func (any *anyMessage) toRoutingMessage(b []byte) RoutingMessage {
+	switch any.Type {
+	case RTM_ADD, RTM_DELETE, RTM_CHANGE, RTM_GET, RTM_LOSING, RTM_REDIRECT, RTM_MISS, RTM_LOCK, RTM_RESOLVE:
+		p := (*RouteMessage)(unsafe.Pointer(any))
+		return &RouteMessage{Header: p.Header, Data: b[SizeofRtMsghdr:any.Msglen]}
+	case RTM_IFINFO:
+		p := (*InterfaceMessage)(unsafe.Pointer(any))
+		return &InterfaceMessage{Header: p.Header, Data: b[SizeofIfMsghdr:any.Msglen]}
+	case RTM_IFANNOUNCE:
+		p := (*InterfaceAnnounceMessage)(unsafe.Pointer(any))
+		return &InterfaceAnnounceMessage{Header: p.Header}
+	case RTM_NEWADDR, RTM_DELADDR:
+		p := (*InterfaceAddrMessage)(unsafe.Pointer(any))
+		return &InterfaceAddrMessage{Header: p.Header, Data: b[SizeofIfaMsghdr:any.Msglen]}
+	}
+	return nil
+}
+
+// InterfaceAnnounceMessage represents a routing message containing
+// network interface arrival and departure information.
+type InterfaceAnnounceMessage struct {
+	Header IfAnnounceMsghdr
+}
+
+func (m *InterfaceAnnounceMessage) sockaddr() (sas []Sockaddr) { return nil }