Start to use struct osmo_auth_vector from gsm_auth_tuple

Rather than having a 'private' structure for kc, sres and rand, we
now finally (with 4 years delay) use osmo_auth_vector from libosmogsm,
which encapsulates authentication vectors that can be either GSM
triplets or UMTS quintuples or a combination of both.

gsm_auth_tuple becomes a wrapper around osmo_auth_vector, adding
use_count and key_seq to it.

key_seq is no longer initialized inside gprs_gsup_messages.c, as there
is no CKSN / key_seq inside the message anyway.  If a usre of the code
needs key_seq, they need to manage it themselves.
diff --git a/openbsc/src/libmsc/db.c b/openbsc/src/libmsc/db.c
index 267b5ef..a23ec89 100644
--- a/openbsc/src/libmsc/db.c
+++ b/openbsc/src/libmsc/db.c
@@ -700,25 +700,25 @@
 	atuple->key_seq = dbi_result_get_ulonglong(result, "key_seq");
 
 	len = dbi_result_get_field_length(result, "rand");
-	if (len != sizeof(atuple->rand))
+	if (len != sizeof(atuple->vec.rand))
 		goto err_size;
 
 	blob = dbi_result_get_binary(result, "rand");
-	memcpy(atuple->rand, blob, len);
+	memcpy(atuple->vec.rand, blob, len);
 
 	len = dbi_result_get_field_length(result, "sres");
-	if (len != sizeof(atuple->sres))
+	if (len != sizeof(atuple->vec.sres))
 		goto err_size;
 
 	blob = dbi_result_get_binary(result, "sres");
-	memcpy(atuple->sres, blob, len);
+	memcpy(atuple->vec.sres, blob, len);
 
 	len = dbi_result_get_field_length(result, "kc");
-	if (len != sizeof(atuple->kc))
+	if (len != sizeof(atuple->vec.kc))
 		goto err_size;
 
 	blob = dbi_result_get_binary(result, "kc");
-	memcpy(atuple->kc, blob, len);
+	memcpy(atuple->vec.kc, blob, len);
 
 	dbi_result_free(result);
 
@@ -759,11 +759,11 @@
 
 	/* Update / Insert */
 	dbi_conn_quote_binary_copy(conn,
-		atuple->rand, sizeof(atuple->rand), &rand_str);
+		atuple->vec.rand, sizeof(atuple->vec.rand), &rand_str);
 	dbi_conn_quote_binary_copy(conn,
-		atuple->sres, sizeof(atuple->sres), &sres_str);
+		atuple->vec.sres, sizeof(atuple->vec.sres), &sres_str);
 	dbi_conn_quote_binary_copy(conn,
-		atuple->kc, sizeof(atuple->kc), &kc_str);
+		atuple->vec.kc, sizeof(atuple->vec.kc), &kc_str);
 
 	if (!upd) {
 		result = dbi_conn_queryf(conn,