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/include/openbsc/sgsn.h b/openbsc/include/openbsc/sgsn.h
index 33bc72f..6b83337 100644
--- a/openbsc/include/openbsc/sgsn.h
+++ b/openbsc/include/openbsc/sgsn.h
@@ -10,7 +10,8 @@
 enum sgsn_auth_policy {
 	SGSN_AUTH_POLICY_OPEN,
 	SGSN_AUTH_POLICY_CLOSED,
-	SGSN_AUTH_POLICY_ACL_ONLY
+	SGSN_AUTH_POLICY_ACL_ONLY,
+	SGSN_AUTH_POLICY_REMOTE
 };
 
 struct sgsn_config {