add more info to the getblock rpc call

This updates namecoin's getblock RPC call to return everything bitcoin's does except for chainwork.
> Please also make it pnext const (or a const reference).

`pnext` isn't a `const` in bitcoin, see [here](https://github.com/bitcoin/bitcoin/blob/master/src/rpcblockchain.cpp#L75).

> Also, why don't you use blockindex->next? (If it is not NULL.) If the block at hand is not in the main chain and you look up the next block by height, it will give a wrong result (besides being unnecessarily complicated). If you use blockindex->next, it won't give any result for blocks not in the main chain. This seems at least better.

In bitcoin, they do `CBlockIndex *pnext = chainActive.Next(blockindex);`. We don't have `chainActive`. We do have `blockindex->pnext`, though, which is more reasonable. I'll update.