sgsn: Integrate subscriber handling into the SGSN

This commit adds a new authorization policy 'remote' and uses
the subscriber cache for authorization when this policy is being used.

Note that there is no remote backend implemented yet. After the
IMSI/IMEI have been acquired, a request would be sent to the remote
peer. The attach/auth-ciph procedure continues when authorization
info has been received from the peer. This means, that
gprs_subscr_update() must be called then to tell the GMM layer
that it can proceed. A later commit will add VTY commands to do this
manually.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 1ee80be..86dd0a2 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -5,7 +5,8 @@
 Testing GMM detach (no MMCTX)
 Testing GMM detach accept (unexpected)
 Testing GMM Status (no MMCTX)
-Testing GMM attach
+Auth policy 'closed': Testing GMM attach
+Auth policy 'remote': Testing GMM attach
 Testing GMM reject
   - Attach Request (invalid MI length)
   - Attach Request (invalid MI type)