simplewallet: don't ask about mining when running a command line

This is likely to be done via a script

src/simplewallet/simplewallet.cpp

@@ -4087,7 +4087,9 @@ bool simple_wallet::init(const boost::program_options::variables_map& vm)
 
   m_wallet->callback(this);
 
-  check_background_mining(password);
+  bool skip_check_backround_mining = !command_line::get_arg(vm, arg_command).empty();
+  if (!skip_check_backround_mining)
+    check_background_mining(password);
 
   if (welcome)
     message_writer(console_color_yellow, true) << tr("If you are new to Monero, type \"welcome\" for a brief overview.");