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/mksysnum_openbsd.pl

@@ -0,0 +1,43 @@
+#!/usr/bin/env perl
+# 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.
+#
+# Generate system call table for OpenBSD from master list
+# (for example, /usr/src/sys/kern/syscalls.master).
+
+use strict;
+
+my $command = "mksysnum_openbsd.pl " . join(' ', @ARGV);
+
+print <<EOF;
+// $command
+// MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
+
+package syscall
+
+const (
+EOF
+
+while(<>){
+	if(/^([0-9]+)\s+STD\s+(NOLOCK\s+)?({ \S+\s+\*?(\w+).*)$/){
+		my $num = $1;
+		my $proto = $3;
+		my $name = $4;
+		$name =~ y/a-z/A-Z/;
+
+		# There are multiple entries for enosys and nosys, so comment them out.
+		if($name =~ /^SYS_E?NOSYS$/){
+			$name = "// $name";
+		}
+		if($name eq 'SYS_SYS_EXIT'){
+			$name = 'SYS_EXIT';
+		}
+
+		print "	$name = $num;  // $proto\n";
+	}
+}
+
+print <<EOF;
+)
+EOF