refactor: Move GetDifficulty out of `rpc/server.h`

It has no business in `rpcserver.h`. Define it in the interface header
of the implementation unit `rpcblockchain` where it is defined.

Also modernize the signature to:

    double GetDifficulty(const CBlockIndex* blockindex = nullptr);

(remove `extern`, replace `NULL` with `nullptr`)

src/Makefile.am

@@ -122,6 +122,7 @@ BITCOIN_CORE_H = \
   protocol.h \
   random.h \
   reverselock.h \
+  rpc/blockchain.h \
   rpc/client.h \
   rpc/protocol.h \
   rpc/server.h \

src/rpc/blockchain.cpp

@@ -3,6 +3,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include "rpc/blockchain.h"
+
 #include "amount.h"
 #include "chain.h"
 #include "chainparams.h"
@@ -42,13 +44,6 @@ static CUpdatedBlock latestblock;
 extern void TxToJSON(const CTransaction& tx, const uint256 hashBlock, UniValue& entry);
 void ScriptPubKeyToJSON(const CScript& scriptPubKey, UniValue& out, bool fIncludeHex);
 
-/**
- * Get the difficulty of the net wrt to the given block index, or the chain tip if
- * not provided.
- *
- * @return A floating point number that is a multiple of the main net minimum
- * difficulty (4295032833 hashes).
- */
 double GetDifficulty(const CBlockIndex* blockindex)
 {
     if (blockindex == NULL)

src/rpc/blockchain.h

@@ -0,0 +1,20 @@
+// Copyright (c) 2017 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_RPC_BLOCKCHAIN_H
+#define BITCOIN_RPC_BLOCKCHAIN_H
+
+class CBlockIndex;
+
+/**
+ * Get the difficulty of the net wrt to the given block index, or the chain tip if
+ * not provided.
+ *
+ * @return A floating point number that is a multiple of the main net minimum
+ * difficulty (4295032833 hashes).
+ */
+double GetDifficulty(const CBlockIndex* blockindex = nullptr);
+
+#endif
+

src/rpc/mining.cpp

@@ -16,6 +16,7 @@
 #include "miner.h"
 #include "net.h"
 #include "pow.h"
+#include "rpc/blockchain.h"
 #include "rpc/server.h"
 #include "txmempool.h"
 #include "util.h"

src/rpc/misc.cpp

@@ -9,6 +9,7 @@
 #include "validation.h"
 #include "net.h"
 #include "netbase.h"
+#include "rpc/blockchain.h"
 #include "rpc/server.h"
 #include "timedata.h"
 #include "util.h"

src/rpc/server.h

@@ -191,7 +191,6 @@ extern std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKe
 
 extern CAmount AmountFromValue(const UniValue& value);
 extern UniValue ValueFromAmount(const CAmount& amount);
-extern double GetDifficulty(const CBlockIndex* blockindex = NULL);
 extern std::string HelpExampleCli(const std::string& methodname, const std::string& args);
 extern std::string HelpExampleRpc(const std::string& methodname, const std::string& args);