[Listener] Improve config file-related warning

Display a warning when the configuration file cannot be read only when
the -f option was used. As stated in comment, the initial plan was to
also display the warning in verbose mode, but at this point of the
program the command-line is not parsed yet, so it's pointless to test
for it.

TODO.t2t

@@ -121,12 +121,6 @@ Work to do in OwlPS
 
 - Refactor and review read_packet().
 
-- Handle better the configuration file reading
-
-  Get rid of the "Cannot open configuration file" error, in quiet mode
-  and when the default configuration file has to be used (no file
-  specified by the user with -f).
-
 - Filter the number of implicit packets per mobile?
 
   Currently, in passive or mixed mode, //all// the received packets are

owlps-listener/owlps-listenerd.c

@@ -418,7 +418,11 @@ int parse_config_file(int argc, char **argv)
       CFG_END()
     } ;
 
-  char *config_file = NULL ; // Configuration file name
+  // Configuration file name
+  char *config_file = NULL ;
+  // True if we are using the default configuration file, false if the
+  // user specified a different one with -f
+  bool default_config_file = false ;
 #endif // OWLPS_LISTENER_USES_CONFIG_FILE
 
   // Option -f specifies a config file, options -h and -V exit the
@@ -460,6 +464,7 @@ int parse_config_file(int argc, char **argv)
   // If -f isn't found, we use the default config file
   if (config_file == NULL)
     {
+      default_config_file = true ;
       config_file =
         malloc((strlen(DEFAULT_CONFIG_FILE) + 1) * sizeof(char)) ;
       if (! config_file)
@@ -476,9 +481,15 @@ int parse_config_file(int argc, char **argv)
   switch (cfg_parse(cfg, config_file))
     {
     case CFG_FILE_ERROR :
-      fprintf(stderr,
-              "Warning! Cannot open configuration file \"%s\": %s.\n",
-              config_file, strerror(errno)) ;
+      /* If we can't open the file, we display a message only if
+       * the user used -f. In verbose mode, it would be nice to
+       * display the message even if the user didn't use -f, but
+       * the command-line options are not parsed yet so the verbose
+       * level is always zero. */
+      if (! default_config_file)
+        fprintf(stderr,
+                "Warning! Cannot open configuration file \"%s\": %s.\n",
+                config_file, strerror(errno)) ;
       break ;
     case CFG_PARSE_ERROR :
       fprintf(stderr,