Change license to BSD 3-clause

[aklomp]

The current license is BSD 2-clause. I don't quite remember why I chose that; I think I wanted something very open while still retaining some rights. But if we change it to BSD 3-clause, it will be easier to integrate code with that license, like @WojciechMula's AVX512 decoder.

We could still integrate code with the BSD 2-clause license, because it's a subset of BSD 3-clause.

The extra third clause is the "non-endorsement" clause. I I agree with the clause (though I don't think it's is very important or relevant for a noncommercial, privately developed library like this), so I see no reason not to add it.

One reason not to change the license might be that it affects downstream users of this library. If their work happens to be under the BSD 2-clause, they would have to add a separate license file for this library. But since this library is typically included as a self-contained source package, I don't think it's a very big concern.