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/rpc/core_rpc_server.cpp

@@ -127,6 +127,7 @@ namespace cryptonote
     res.incoming_connections_count = total_conn - res.outgoing_connections_count;
     res.white_peerlist_size = m_p2p.get_peerlist_manager().get_white_peers_count();
     res.grey_peerlist_size = m_p2p.get_peerlist_manager().get_gray_peers_count();
+    res.testnet = m_testnet;
     res.status = CORE_RPC_STATUS_OK;
     return true;
   }