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/syscall_bsd_test.go

@@ -0,0 +1,34 @@
+// Copyright 2014 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.
+
+// +build darwin dragonfly freebsd openbsd
+
+package syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+const MNT_WAIT = 1
+
+func TestGetfsstat(t *testing.T) {
+	n, err := syscall.Getfsstat(nil, MNT_WAIT)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	data := make([]syscall.Statfs_t, n)
+	n, err = syscall.Getfsstat(data, MNT_WAIT)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	empty := syscall.Statfs_t{}
+	for _, stat := range data {
+		if stat == empty {
+			t.Fatal("an empty Statfs_t struct was returned")
+		}
+	}
+}