nat: Allow a BSC to have multiple LACs

Make it possible that one BSC is serving multiple
cells. Introduce a list of lacs, add functions to
manipulate the lists. The current test cases for
paging by lac continue to work.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 48407b9..f5eca80 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -753,8 +753,8 @@
 			bsc->authenticated = 1;
 			bsc->cfg = conf;
 			bsc_del_timer(&bsc->id_timeout);
-			LOGP(DNAT, LOGL_NOTICE, "Authenticated bsc nr: %d lac: %d on fd %d\n",
-			     conf->nr, conf->lac, bsc->write_queue.bfd.fd);
+			LOGP(DNAT, LOGL_NOTICE, "Authenticated bsc nr: %d on fd %d\n",
+			     conf->nr, bsc->write_queue.bfd.fd);
 			start_ping_pong(bsc);
 			return;
 		}