add struct gprs_subscr, separating gprs from gsm_subscriber

Prepare for replacing gsm_subscriber with vlr_subscriber. vlr_subscriber will
not make sense to be used in gprs, so have a dedicated GPRS subscriber struct.
(Could change if the gprs code were to use libvlr; is currently independent).

Related: OS#1592
Change-Id: Ia8b391ee009c8545763cba04505be3947835120e
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index c7a53b9..f38d730 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -1,5 +1,13 @@
 Testing LLME allocations
 Testing core subscriber data API
+llist_count(gprs_subscribers) == 0
+llist_count(gprs_subscribers) == 1
+llist_count(gprs_subscribers) == 1
+llist_count(gprs_subscribers) == 2
+llist_count(gprs_subscribers) == 3
+llist_count(gprs_subscribers) == 2
+llist_count(gprs_subscribers) == 1
+llist_count(gprs_subscribers) == 0
 Testing authentication triplet handling
 Testing subscriber GSUP handling
 Testing GMM detach