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.
{
        nWalletVersion = FEATURE_BASE;
        nWalletMaxVersion = FEATURE_BASE;
        nMasterKeyMaxID = 0;
        pwalletdbEncryption = nullptr;
        nOrderPosNext = 0;
        nAccountingEntryNumber = 0;
        nNextResend = 0;
        nLastResend = 0;
        m_max_keypool_index = 0;
        nTimeFirstKey = 0;
        fBroadcastTransactions = false;
        nRelockTime = 0;
        fAbortRescan = false;
        fScanningWallet = false;
    }
    std::map<uint256, CWalletTx> mapWallet;
    std::list<CAccountingEntry> laccentries;
    typedef std::pair<CWalletTx*, CAccountingEntry*> TxPair;
    typedef std::multimap<int64_t, TxPair > TxItems;
    TxItems wtxOrdered;
    int64_t nOrderPosNext;
    uint64_t nAccountingEntryNumber;
    std::map<uint256, int> mapRequestCount;
    std::map<CTxDestination, CAddressBookData> mapAddressBook;
    std::set<COutPoint> setLockedCoins;
    const CWalletTx* GetWalletTx(const uint256& hash) const;
    
    bool CanSupportFeature(enum WalletFeature wf) const { AssertLockHeld(cs_wallet); return nWalletMaxVersion >= wf; }