MSC pooling: make NRI mappings VTY configurable

Use the osmo_nri_ranges API to manage each MSC's NRI ranges by VTY
configuration.

Change-Id: I6c251f2744d7be26fc4ad74adefc96a6a3fe08b0
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index bc4c017..38047d2 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1716,6 +1716,9 @@
 
 	/* Don't refuse to start with mutually exclusive codec settings */
 	bool allow_unusable_timeslots;
+
+	uint8_t nri_bitlen;
+	struct osmo_nri_ranges *null_nri_ranges;
 };
 
 struct gsm_audio_support {