fix: populate ldns submodule and add autotools to LDNS build stage

- Re-cloned zonemaster-ldns with --recurse-submodules so the bundled
  ldns C library source (including Changelog and configure.ac) is present
- Added autoconf, automake, libtool to Dockerfile.backend ldns-build stage
  so libtoolize + autoreconf can generate ldns/configure during make

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster-ldns/ldns/masterdont/zinfo.h

@@ -0,0 +1,112 @@
+/* zinfo.h - zone info */
+
+#ifndef ZINFO_H
+#define ZINFO_H
+struct zone_entry_t;
+
+/**
+ * Store zone content information
+ * zone identity is stored in zone_entry_t.
+ */
+struct zinfo_t {
+	/** 
+	 * directory to use for disk storage 
+	 *	o zone.<name>.index  --  oldest SOA number in dir
+	 *	o zone.<name>.ixfr.<soa>  --  text format ixfr to next version.
+	 *	o zone.<name>.full.<soa>  --  text format full zone file.
+	 * The full file may be omitted for many versions. At least one.
+	 */
+	char* dir;
+
+	/** is the zone present at all (or empty, no data) */
+	int is_present;
+	/** latest serial number */
+	uint32_t last_serial;
+	/** latest SOA (also present in complete zone) plus rrsigs */
+	ldns_rr_list* last_soa;
+
+	/** tree of versions, sorted by serial and contains IXFRs.
+	 * the last serial number is not in here, but kept as complete zone. */
+	ldns_rbtree_t vs;
+
+	/** for last version, the complete zone. zdomain_t sorted by name */
+	ldns_rbtree_t zone;
+};
+
+/**
+ * A zone version
+ */
+struct zversion_t {
+	/** rbtree node, sorted by serial */
+	ldns_rbnode_t node;
+	/** serial number */
+	uint32_t serial;
+	/** next serial number */
+	uint32_t next_serial;
+	/** ixfr contents */
+	ldns_rr_list* ixfr;
+};
+
+/**
+ * A domain name in the complete zone tree
+ */
+struct zdomain_t {
+	/** rbtree node */
+	ldns_rbnode_t node;
+	/** rdf name */
+	ldns_rdf* name;
+	/** the rrsets present, sorted by type */
+	ldns_rbtree_t rrsets;
+};
+
+/**
+ * An rrset at a name in the complete zone tree
+ */
+struct zrrset_t {
+	/** rbtree node */
+	ldns_rbnode_t node;
+	/** rr set */
+	ldns_rr_list* list;
+};
+
+/**
+ * Create a zinfo. No data, empty.
+ */
+struct zinfo_t* zinfo_create(void);
+
+/**
+ * Delete a zinfo
+ */
+void zinfo_delete(struct zinfo_t* zinfo);
+
+/**
+ * Read zone info from stable storage (disk)
+ */
+int zinfo_read(struct zone_entry_t* entry);
+
+/** Get a pointer to a static buffer with filename */
+const char* zinfo_index_name(struct zone_entry_t* entry);
+/** Get a pointer to a static buffer with filename */
+const char* zinfo_ixfr_name(struct zone_entry_t* entry, uint32_t soa);
+/** Get a pointer to a static buffer with filename */
+const char* zinfo_full_name(struct zone_entry_t* entry, uint32_t soa);
+
+int zinfo_get_zone_diff(struct zone_entry_t* entry, uint32_t serial_from, 
+	uint32_t serial_to, ldns_rr_list** rr_remove, ldns_rr_list** rr_add, 
+	ldns_rr** soa_from, ldns_rr** soa_to);
+
+void zinfo_get_zone_full(struct zone_entry_t* entry, uint32_t serial,
+	ldns_zone** z);
+
+/* ----------------------- zversion ---------------------- */
+void zversion_delete(struct zversion_t* v);
+struct zversion_t* zversion_read(struct zone_entry_t* entry, uint32_t serial);
+
+/* ----------------------- zdomain ---------------------- */
+void zdomain_delete(struct zdomain_t* d);
+int zfull_read(struct zone_entry_t* entry, uint32_t serial);
+
+/* ----------------------- zrrset ---------------------- */
+void zrrset_delete(struct zrrset_t* r);
+
+#endif