windows: add NewLazySystemDLL and use it in zsyscall_windows.go

If we want new secure DLL approach to be adopted, we should make
conversion as simple as possible to explain and implement.
I think that replacing

syscall.NewLazyDLL(...) -> windows.NewLazySystemDLL(...)

is easier than

syscall.NewLazyDLL(...) -> &windows.LazyDLL{Name: ..., System: true}

So I propose we introduce convenience function NewLazySystemDLL.

$GOROOT/src/mksyscall_windows.go changes in the following CL.

Change-Id: If3432aff301c347cb355e4e837834696191b2219
Reviewed-on: https://go-review.googlesource.com/21592
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/dll_windows.go

@@ -235,6 +235,13 @@ func NewLazyDLL(name string) *LazyDLL {
 	return &LazyDLL{Name: name}
 }
 
+// NewLazySystemDLL is like NewLazyDLL, but will only
+// search Windows System directory for the DLL if name is
+// a base name (like "advapi32.dll").
+func NewLazySystemDLL(name string) *LazyDLL {
+	return &LazyDLL{Name: name, System: true}
+}
+
 // A LazyProc implements access to a procedure inside a LazyDLL.
 // It delays the lookup until the Addr method is called.
 type LazyProc struct {