dns-domain: use dns_name_concat()

`dns_name_normalize()` is an alias of `dns_name_concat()`.
For consistency with `dns_name_is_valid_ldh()`, let's use
`dns_name_concat()`.

src/shared/dns-domain.h

@@ -40,8 +40,8 @@ static inline int dns_name_normalize(const char *s, DNSLabelFlags flags, char **
 static inline int dns_name_is_valid(const char *s) {
         int r;
 
-        /* dns_name_normalize() verifies as a side effect */
-        r = dns_name_normalize(s, 0, NULL);
+        /* dns_name_concat() verifies as a side effect */
+        r = dns_name_concat(s, NULL, 0, NULL);
         if (r == -EINVAL)
                 return 0;
         if (r < 0)