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/shared/fdset.c

@@ -127,7 +127,7 @@ int fdset_new_fill(FDSet **_s) {
         while ((de = readdir(d))) {
                 int fd = -1;
 
-                if (ignore_file(de->d_name))
+                if (hidden_file(de->d_name))
                         continue;
 
                 r = safe_atoi(de->d_name, &fd);