teach fiche about binding to a specific address

Add the -L <listen_addr> option which permits fiche to bind to a
specific local address rather than INADDR_ANY.

fiche.h

@@ -43,6 +43,11 @@ typedef struct Fiche_Settings {
      */
     char *output_dir_path;
 
+    /**
+     * @brief Address on which fiche is waiting for connections
+     */
+    char *listen_addr;
+
     /**
      * @brief Port on which fiche is waiting for connections
      */