MDEV-19015: mysql_plugin doesn't read all server option groups

Closes #1262

client/mysql_plugin.c

@@ -321,7 +321,7 @@ static int get_default_values()
   int ret= 0;
   FILE *file= 0;
 
-  bzero(tool_path, FN_REFLEN);
+  memset(tool_path, 0, FN_REFLEN);
   if ((error= find_tool("my_print_defaults" FN_EXEEXT, tool_path)))
     goto exit;
   else
@@ -334,9 +334,9 @@ static int get_default_values()
       char *format_str= 0;
   
       if (has_spaces(tool_path) || has_spaces(defaults_file))
-        format_str = "\"%s mysqld > %s\"";
+        format_str = "\"%s --mysqld > %s\"";
       else
-        format_str = "%s mysqld > %s";
+        format_str = "%s --mysqld > %s";
   
       snprintf(defaults_cmd, sizeof(defaults_cmd), format_str,
                add_quotes(tool_path), add_quotes(defaults_file));
@@ -347,7 +347,7 @@ static int get_default_values()
     }
 #else
     snprintf(defaults_cmd, sizeof(defaults_cmd),
-             "%s mysqld > %s", tool_path, defaults_file);
+             "%s --mysqld > %s", tool_path, defaults_file);
 #endif
 
     /* Execute the command */