rpc: fix start_mining and status RPC crashes

They check whether they're running on testnet by accessing the
m_rpc_server object, which does not exist when in RPC mode.

Also, fix hard_fork_info being called with the wrong API.

src/daemon/rpc_command_executor.h

@@ -95,7 +95,7 @@ public:
 
   bool print_transaction_pool_short();
 
-  bool start_mining(cryptonote::account_public_address address, uint64_t num_threads);
+  bool start_mining(cryptonote::account_public_address address, uint64_t num_threads, bool testnet);
 
   bool stop_mining();