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.
/**
     * populate vCoins with vector of available COutputs.
     */
    void AvailableCoins(std::vector<COutput>& vCoins, bool fOnlySafe=true, const CCoinControl *coinControl = nullptr, const CAmount& nMinimumAmount = 1, const CAmount& nMaximumAmount = MAX_MONEY, const CAmount& nMinimumSumAmount = MAX_MONEY, const uint64_t& nMaximumCount = 0, const int& nMinDepth = 0, const int& nMaxDepth = 9999999) const;
    /**
     * Return list of available coins and locked coins grouped by non-change output address.
     */
    std::map<CTxDestination, std::vector<COutput>> ListCoins() const;
    /**
     * Find non-change parent output.
     */
    const CTxOut& FindNonChangeParentOutput(const CTransaction& tx, int output) const;
    /**
     * Shuffle and select coins until nTargetValue is reached while avoiding
     * small change; This method is stochastic for some inputs and upon
     * completion the coin set and corresponding actual target value is
     * assembled
     */
    bool SelectCoinsMinConf(const CAmount& nTargetValue, int nConfMine, int nConfTheirs, uint64_t nMaxAncestors, std::vector<COutput> vCoins, std::set<CInputCoin>& setCoinsRet, CAmount& nValueRet) const;
    bool IsSpent(const uint256& hash, unsigned int n) const;
    bool IsLockedCoin(uint256 hash, unsigned int n) const;
    void LockCoin(const COutPoint& output);
    void UnlockCoin(const COutPoint& output);
    void UnlockAllCoins();
    void ListLockedCoins(std::vector<COutPoint>& vOutpts) const;
    /*
     * Rescan abort properties
     */
    void AbortRescan() { fAbortRescan = true; }