wallet: make it possible to disable transaction broadcast

This is an advanced feature which will disable any kind of automatic
transaction broadcasting in the wallet. This gives the user full control
of how the transaction is sent.

For example they can broadcast new transactions through some other
mechanism themselves, after getting the transaction hex through `gettransaction`.

This just adds the option `-walletbroadcast=<0,1>`. Right now these
transactions will get the status

    Status: conflicted, has not been successfully broadcast yet

They shouldn't be shown as conflicted at all (`walletconflicts` is empty). This status
will go away when the transaction is received through the network.

src/wallet/wallet.h

@@ -455,6 +455,7 @@ private:
 
     int64_t nNextResend;
     int64_t nLastResend;
+    bool fBroadcastTransactions;
 
     /**
      * Used to keep track of spent outpoints, and
@@ -518,6 +519,7 @@ public:
         nNextResend = 0;
         nLastResend = 0;
         nTimeFirstKey = 0;
+        fBroadcastTransactions = false;
     }
 
     std::map<uint256, CWalletTx> mapWallet;
@@ -723,6 +725,11 @@ public:
 
     /** Watch-only address added */
     boost::signals2::signal<void (bool fHaveWatchOnly)> NotifyWatchonlyChanged;
+
+    /** Inquire whether this wallet broadcasts transactions. */
+    bool GetBroadcastTransactions() const { return fBroadcastTransactions; }
+    /** Set whether this wallet broadcasts transactions. */
+    void SetBroadcastTransactions(bool broadcast) { fBroadcastTransactions = broadcast; }
 };
 
 /** A key allocated from the key pool. */