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_netbsd.pl

@@ -0,0 +1,51 @@
+#!/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_netbsd.pl " . join(' ', @ARGV);
+
+print <<EOF;
+// $command
+// MACHINE GENERATED BY THE ABOVE COMMAND; DO NOT EDIT
+
+package syscall
+
+const (
+EOF
+
+my $line = '';
+while(<>){
+	if($line =~ /^(.*)\\$/) {
+		# Handle continuation
+		$line = $1;
+		$_ =~ s/^\s+//;
+		$line .= $_;
+	} else {
+		# New line
+		$line = $_;
+	}
+	next if $line =~ /\\$/;
+	if($line =~ /^([0-9]+)\s+((STD)|(NOERR))\s+(RUMP\s+)?({\s+\S+\s*\*?\s*\|(\S+)\|(\S*)\|(\w+).*\s+})(\s+(\S+))?$/) {
+		my $num = $1;
+		my $proto = $6;
+		my $compat = $8;
+		my $name = "$7_$9";
+
+		$name = "$7_$11" if $11 ne '';
+		$name =~ y/a-z/A-Z/;
+
+		if($compat eq '' || $compat eq '30' || $compat eq '50') {
+			print "	$name = $num;  // $proto\n";
+		}
+	}
+}
+
+print <<EOF;
+)
+EOF