Detect old option specification fail informatively.

Print the information message to std::cout so it gets printed on the tmux
status line so users of 2.X know how to update their configuration.

CMakeLists.txt

@@ -115,8 +115,13 @@ if(BUILD_TESTING)
     COMMAND tmux-mem-cpu-load --graph_lines -2
     )
 
+  add_test(NAME old_option_specification
+    COMMAND tmux-mem-cpu-load 2 8
+    )
+
   set_tests_properties(usage
     invalid_status_interval
     invalid_graph_lines
+    old_option_specification
     PROPERTIES WILL_FAIL TRUE)
 endif()

tmux-mem-cpu-load.cc

@@ -159,6 +159,14 @@ int main( int argc, char** argv )
         return EXIT_FAILURE;
     }
   }
+  // Detect old option specification and return and error message.
+  if( argc > optind )
+  {
+    std::cout <<
+      "The interval and graph lines options are now specified with flags.\n\n";
+    print_help();
+    return EXIT_FAILURE;
+  }
 
   std::cout << mem_string( use_colors ) << ' '
     << cpu_string( cpu_usage_delay, graph_lines, use_colors ) << ' '