windows: add support for NTSTATUS values

The native NT API returns error values from a different namespace as the
usual Win32 one. This means it needs to be typed differently. This
commit adds broad support for using NTSTATUS values in a new type called
NTStatus.

First we add the type as a basic uint32. Then we add all of the
predefined constants from ntstatus.h, by augmenting mkerrors.bash to do
automatic extraction. There's a convenece way to convert an NT error to
a Win32 error, so we add the NTStatus.Errno() function. Since NTStatus
is an error type, we define an Error() function that returns a string by
asking ntdll.dll for its contents, in the exact same way that
syscall.Errno.Error() does, by calling FormatMessage. Since functions
need to actually use this, we add the rule that if a `//sys` declaration
returns an error value called "ntstatus", then the type underlying the
error interface is an NTStatus instead of an Errno. Finally we fix one
function that was returning an error interface of an Errno rather than
an NTStatus.

Change-Id: I06296b9563bbec526759d12a19f13ac6ad46dcc3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/297330
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

windows/syscall_windows_test.go

@@ -486,3 +486,11 @@ func TestIsWow64Process2(t *testing.T) {
 		t.Errorf("IsWow64Process2 is wrong: want %v have %v", runtime.GOARCH, processMachine)
 	}
 }
+
+func TestNTStatusString(t *testing.T) {
+	want := "The name limit for the local computer network adapter card was exceeded."
+	got := windows.STATUS_TOO_MANY_NAMES.Error()
+	if want != got {
+		t.Errorf("NTStatus.Error did not return an expected error string - want %q; got %q", want, got)
+	}
+}