Built against `master` branch 801dd40666d1e6009920ad3ff755c7bb993b2a62:

If I have no wallet.dat file and run:

    bitcoind -usehd=0

A new wallet.dat file is created and an HD seed is assigned.

I see an error message in the source:

    "Error loading %s: You can't disable HD on an already existing HD wallet or create new non-HD wallets."

but that doesn't get shown because `fFirstRun` is true, and so the code which checks for `-usehd` values doesn't run.

If it's no longer possible to create non-HD wallets, specifying -usehd=0 when creating a wallet should be an error, and not get silently ignored.
if (keypool.fInternal != fReturningInternal) {
            throw std::runtime_error(std::string(__func__) + ": keypool entry misclassified");
        }
        assert(keypool.vchPubKey.IsValid());
        m_pool_key_to_index.erase(keypool.vchPubKey.GetID());
        LogPrintf("keypool reserve %d\n", nIndex);
    }
}
void CWallet::KeepKey(int64_t nIndex)
{
    
    CWalletDB walletdb(*dbw);
    walletdb.ErasePool(nIndex);
    LogPrintf("keypool keep %d\n", nIndex);
}
void CWallet::ReturnKey(int64_t nIndex, bool fInternal, const CPubKey& pubkey)
{
    
    {
        LOCK(cs_wallet);
        if (fInternal) {
            setInternalKeyPool.insert(nIndex);
        } else {
            setExternalKeyPool.insert(nIndex);
        }
        m_pool_key_to_index[pubkey.GetID()] = nIndex;
    }
    LogPrintf("keypool return %d\n", nIndex);
}
bool CWallet::GetKeyFromPool(CPubKey& result, bool internal)