nat: Create a Paging Group that BSCs can refer to

Introduce a paging group that a BSC can refer to and is used
during the LAC lookup. This way paging can be flooded through
the network and just filtered at the last element in the core.
diff --git a/openbsc/src/libcommon/common_vty.c b/openbsc/src/libcommon/common_vty.c
index f14dbca..5b4b296 100644
--- a/openbsc/src/libcommon/common_vty.c
+++ b/openbsc/src/libcommon/common_vty.c
@@ -83,6 +83,9 @@
 			vty->index = bsc_config->nat;
 		}
 		break;
+	case PGROUP_NODE:
+		vty->node = NAT_NODE;
+		break;
 	case MSC_NODE:
 		vty->node = CONFIG_NODE;
 		break;
@@ -139,6 +142,9 @@
 			vty->index = bsc_config->nat;
 		}
 		break;
+	case PGROUP_NODE:
+		vty->node = NAT_NODE;
+		break;
 	case MGCP_NODE:
 	case GBPROXY_NODE:
 	case SGSN_NODE:
@@ -189,6 +195,7 @@
 	case VTY_NODE:
 	case NAT_NODE:
 	case NAT_BSC_NODE:
+	case PGROUP_NODE:
 	case MSC_NODE:
 		vty_config_unlock(vty);
 		vty->node = ENABLE_NODE;