miner: bugfix: fix duplicate weight reservation in block assembler

- This commit renamed coinbase_max_additional_weight to block_reserved_weight.

- Also clarify that the reservation is for block header, transaction count
  and coinbase transaction.

src/init.cpp

@@ -19,6 +19,7 @@
 #include <common/args.h>
 #include <common/system.h>
 #include <consensus/amount.h>
+#include <consensus/consensus.h>
 #include <deploymentstatus.h>
 #include <hash.h>
 #include <httprpc.h>