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.
{
    CWalletDB walletdb(*dbw);
    CAccount account;
    walletdb.ReadAccount(strAccount, account);
    if (!bForceNew) {
        if (!account.vchPubKey.IsValid())
            bForceNew = true;
        else {
            
            CScript scriptPubKey = GetScriptForDestination(account.vchPubKey.GetID());
            for (std::map<uint256, CWalletTx>::iterator it = mapWallet.begin();
                 it != mapWallet.end() && account.vchPubKey.IsValid();
                 ++it)
                for (const CTxOut& txout : (*it).second.tx->vout)
                    if (txout.scriptPubKey == scriptPubKey) {
                        bForceNew = true;
                        break;
                    }
        }
    }
    
    if (bForceNew) {
        if (!GetKeyFromPool(account.vchPubKey, false))
            return false;
        SetAddressBook(account.vchPubKey.GetID(), strAccount, "receive");
        walletdb.WriteAccount(strAccount, account);
    }
    pubKey = account.vchPubKey;