wallet: avoid reuse flags

Add m_avoid_address_reuse flag to coin control object.
Add avoid_reuse wallet flag and accompanying strings/caveats.

src/wallet/coincontrol.cpp

@@ -13,6 +13,7 @@ void CCoinControl::SetNull()
     fAllowOtherInputs = false;
     fAllowWatchOnly = false;
     m_avoid_partial_spends = gArgs.GetBoolArg("-avoidpartialspends", DEFAULT_AVOIDPARTIALSPENDS);
+    m_avoid_address_reuse = false;
     setSelected.clear();
     m_feerate.reset();
     fOverrideFeeRate = false;