dns-domain: introduce dns_name_hash_ops_free

Preparation for later commits.

src/shared/dns-domain.c

@@ -527,7 +527,18 @@ int dns_name_compare_func(const char *a, const char *b) {
         }
 }
 
-DEFINE_HASH_OPS(dns_name_hash_ops, char, dns_name_hash_func, dns_name_compare_func);
+DEFINE_HASH_OPS(
+        dns_name_hash_ops,
+        char,
+        dns_name_hash_func,
+        dns_name_compare_func);
+
+DEFINE_HASH_OPS_WITH_KEY_DESTRUCTOR(
+        dns_name_hash_ops_free,
+        char,
+        dns_name_hash_func,
+        dns_name_compare_func,
+        free);
 
 int dns_name_equal(const char *x, const char *y) {
         int r, q;

src/shared/dns-domain.h

@@ -67,6 +67,7 @@ static inline bool dns_name_is_empty(const char *s) {
 void dns_name_hash_func(const char *s, struct siphash *state);
 int dns_name_compare_func(const char *a, const char *b);
 extern const struct hash_ops dns_name_hash_ops;
+extern const struct hash_ops dns_name_hash_ops_free;
 
 int dns_name_between(const char *a, const char *b, const char *c);
 int dns_name_equal(const char *x, const char *y);