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/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index 1acd6f2..fe41e75 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -1132,7 +1132,7 @@
 int my_subscr_request_auth_info_real_auth(struct sgsn_mm_ctx *mmctx)
 {
 	struct gsm_auth_tuple at = {
-		.sres = {0x51, 0xe5, 0x51, 0xe5},
+		.vec.sres = {0x51, 0xe5, 0x51, 0xe5},
 		.key_seq = 0
 	};