D-GSM 1/n: add mslookup server in osmo-hlr

Implement the mslookup server to service remote mslookup requests.

This patch merely adds the logic to answer incoming mslookup requests, an
actual method to receive requests (mDNS) follows in a subsequent patch.

- API to configure service names and addresses for the local site (per MSC).
- determine whether a subscriber is on a local MSC
  (checking the local proxy will be added in subsequent patch that adds proxy
  capability).
- VTY config follows in a subsequent patch.

For a detailed overview of the D-GSM and mslookup related files, please see the
elaborate comment at the top of mslookup.c (already added in an earlier patch).

Change-Id: Ife4a61d71926d08f310a1aeed9d9f1974f64178b
diff --git a/include/osmocom/hlr/hlr.h b/include/osmocom/hlr/hlr.h
index 5885600..1269994 100644
--- a/include/osmocom/hlr/hlr.h
+++ b/include/osmocom/hlr/hlr.h
@@ -67,6 +67,13 @@
 	/* Bitmask of DB_SUBSCR_FLAG_* */
 	uint8_t subscr_create_on_demand_flags;
 	unsigned int subscr_create_on_demand_rand_msisdn_len;
+
+	struct {
+		struct {
+			uint32_t local_attach_max_age;
+			struct llist_head local_site_services;
+		} server;
+	} mslookup;
 };
 
 extern struct hlr *g_hlr;