wallet: new option to start background mining

The setup-background-mining option can be used to select
background mining when a wallet loads. The user will be asked
the first time the wallet is created.

src/wallet/wallet_rpc_server.h

@@ -252,6 +252,8 @@ namespace tools
 
       bool validate_transfer(const std::list<wallet_rpc::transfer_destination>& destinations, const std::string& payment_id, std::vector<cryptonote::tx_destination_entry>& dsts, std::vector<uint8_t>& extra, bool at_least_one_destination, epee::json_rpc::error& er);
 
+      void check_background_mining();
+
       wallet2 *m_wallet;
       std::string m_wallet_dir;
       tools::private_file rpc_login_file;