C#: Invalid code for unsigned enums
When compiling the C# code generated from the enum `section_flags` in [this ksy file](https://github.com/smx-smx/reko/blob/aae2f69e03dade1f1346b562006adca654f4785b/src/ImageLoaders/Coff/ksy/ecoff.ksy), we get the following:

```
Error	CS0266	Cannot implicitly convert type 'uint' to 'int'. An explicit conversion exists (are you missing a cast?)
```

The problem is that the generated enum class should have the following form:
```csharp
public enum SectionFlags : uint
```

because the source enum uses unsigned values

![screenshot](https://user-images.githubusercontent.com/1978844/111387424-fb638680-86ad-11eb-9d03-ef8bfd093c63.png)
