sgsn: Support subscriber based authentication

This commit mainly extends sgsn_auth.c to use and support the
auth_state SGSN_AUTH_AUTHENTICATE. It will be activated when IMSI and
IMEI are available, authentication is required
(subscr->sgsn_data->authenticate is set), but the MM context is not
marked as authenticated. If the state has been set to
SGSN_AUTH_AUTHENTICATE and sgsn_auth_update() is called, the GMM
layer will be informed by invoking gsm0408_gprs_authenticate().

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 4b061a5..7d739ad 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -7,6 +7,7 @@
 Testing GMM Status (no MMCTX)
 Auth policy 'closed': Testing GMM attach
 Auth policy 'remote': Testing GMM attach
+Auth policy 'remote', auth faked: Testing GMM attach
 Testing GMM reject
   - Attach Request (invalid MI length)
   - Attach Request (invalid MI type)