fs-util: unify code we use to check if dirent's d_name is "." or ".."

We use different idioms at different places. Let's replace this is the
one true new idiom, that is even a bit faster...

src/basic/socket-util.c

@@ -877,7 +877,7 @@ bool ifname_valid(const char *p) {
         if (strlen(p) >= IFNAMSIZ)
                 return false;
 
-        if (STR_IN_SET(p, ".", ".."))
+        if (dot_or_dot_dot(p))
                 return false;
 
         while (*p) {