blockheaders include block height, nodes validate it

The block height validation is gated by the startup option
`con_blockheightinheader`, accesible in custom chains only
using option `chain=<name>`. Only if set to true will this
field be (de)serialized and evaluated in consensus logic.
Otherwise the field is simply ignored, other than being
stored in memory as the default value 0.

src/chainparams.cpp

@@ -435,6 +435,10 @@ class CCustomParams : public CRegTestParams {
         // No subsidy for custom chains by default
         consensus.genesis_subsidy = args.GetArg("-con_blocksubsidy", 0);
 
+        // Note: This global is needed to avoid circular dependency
+        // Defaults to true for custom chains.
+        g_con_blockheightinheader = gArgs.GetBoolArg("-con_blockheightinheader", true);
+
         // All non-zero coinbase outputs must go to this scriptPubKey
         std::vector<unsigned char> man_bytes = ParseHex(gArgs.GetArg("-con_mandatorycoinbase", ""));
         consensus.mandatory_coinbase_destination = CScript(man_bytes.begin(), man_bytes.end()); // Blank script allows any coinbase destination