windows/svc: add new package to help create and manage Windows services

Change-Id: I58bb446aaa387b31d8a9ff4217793a170b96a7e2
Reviewed-on: https://go-review.googlesource.com/9104
Reviewed-by: Rob Pike <r@golang.org>

windows/svc/go13.go

@@ -0,0 +1,31 @@
+// 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 windows
+// +build go1.3
+
+package svc
+
+import "unsafe"
+
+const ptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const
+
+// Should be a built-in for unsafe.Pointer?
+func add(p unsafe.Pointer, x uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(p) + x)
+}
+
+// funcPC returns the entry PC of the function f.
+// It assumes that f is a func value. Otherwise the behavior is undefined.
+func funcPC(f interface{}) uintptr {
+	return **(**uintptr)(add(unsafe.Pointer(&f), ptrSize))
+}
+
+// from sys_386.s and sys_amd64.s
+func servicectlhandler(ctl uint32) uintptr
+func servicemain(argc uint32, argv **uint16)
+
+func getServiceMain(r *uintptr) {
+	*r = funcPC(servicemain)
+}