Add AllowUnsafeBlocks to all projects

[AArnott]

As an increasing number of PInvoke assemblies need to allow unsafe blocks, and we're p/invoking into native code, after all, it makes sense to just allow it for all projects.

Let's add it to EnlistmentInfo.props, and remove the explicit property definition from the individual projects that already have them added.

<AllowUnsafeBlocks>true</AllowUnsafeBlocks>