[BSC] Allow manual override of neighbor cell list

So far, OpenBSC simply assumed that all BTS's configured in openbsc.cfg are
neighbors of each other.  While this is true for small site installations,
it is definitely not true in most real world cases.  We now have the
following new commands at the 'configure bts' level:

'neighbor-list mode (auto|manual)' for selecting the mode

'neighbor-list (add|del) arfcn <0-1024>'
	for adding/deleting ARFCN in manual mode
diff --git a/openbsc/src/gsm_data.c b/openbsc/src/gsm_data.c
index 705c107..5ae5258 100644
--- a/openbsc/src/gsm_data.c
+++ b/openbsc/src/gsm_data.c
@@ -207,6 +207,8 @@
 	bts->num_trx = 0;
 	INIT_LLIST_HEAD(&bts->trx_list);
 	bts->ms_max_power = 15;	/* dBm */
+
+	bts->neigh_list_manual_mode = 0;
 	bts->si_common.cell_sel_par.cell_resel_hyst = 2; /* 4 dB */
 	bts->si_common.cell_sel_par.rxlev_acc_min = 0;
 	bts->si_common.neigh_list.data = bts->si_common.data.neigh_list;