unix, windows: add type aliases to std syscall for Signal, Errno, SysProcAttr

To simplify the pkg syscall docs.

Change-Id: I3ed0bfc59564ada280141c4662a56e141d197e1f
Reviewed-on: https://go-review.googlesource.com/127656
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/aliases.go

+// Copyright 2018 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 linux netbsd openbsd solaris
+// +build go1.9
+
+package unix
+
+import "syscall"
+
+type Signal = syscall.Signal
+type Errno = syscall.Errno
+type SysProcAttr = syscall.SysProcAttr

windows/aliases.go

+// Copyright 2018 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 windows
+// +build go1.9
+
+package windows
+
+import "syscall"
+
+type Errno = syscall.Errno
+type SysProcAttr = syscall.SysProcAttr