util: rename ignore_file() to hidden_file()

hidden_file() is a bit more precise, since dot files usually shouldn't
be ignored, but certainly be considered hidden.

src/core/manager.c

@@ -1028,7 +1028,7 @@ static void manager_build_unit_path_cache(Manager *m) {
                 while ((de = readdir(d))) {
                         char *p;
 
-                        if (ignore_file(de->d_name))
+                        if (hidden_file(de->d_name))
                                 continue;
 
                         p = strjoin(streq(*i, "/") ? "" : *i, "/", de->d_name, NULL);