Implement support for optional allowed downloading on pure servers.

[dGr8LookinSparky]

@Chomenor , how would we handle downloading for pure servers (although I suppose optional downloading can work for semipure and unpure as well) with allowed optional pk3 files? In such a case some pk3 files might be required for the client to have, while other pk3 files are optional for the client to have (like optional approved skins), but the client wouldn't be allowed to use any other pk3 file. So I think the download prompt would have two sections:

• One section listing all of the required files that you don't have and all will be accepted if you click the download button, while all will be rejected if you click disconnect.

• Another section itemizing all of the optional allowed files, and there might be a check box next to each, where you would check the ones you want to download, and the ones not checked won't be downloaded.