gprs: Add subscriber functions to create/handle GSUP messages

This patch extends gprs_subscr_query_auth_info and
gprs_subscr_location_update to create GSUP messages with the help of
a static gprs_subscr_tx_gsup_message function. A corresponding
gprs_subscr_rx_gsup_message is added which takes a messages, gets the
subscr, and updates it accordingly.

Sponsored-by: On-Waves ehf

[hfreyther: Added a msgb_free gprs_subscr_tx_gsup_message]
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index e54991c..b118328 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -1,6 +1,7 @@
 Testing LLME allocations
 Testing core subscriber data API
 Testing authentication triplet handling
+Testing subcriber GSUP handling
 Testing GMM detach
 Testing GMM detach (power off)
 Testing GMM detach (no MMCTX)