ctrl: Move the lookup into a separate file in preparation for GPRS

For GPRS the look-up via bts/trx does not make any sense and would
introduce bad depdencies for the SGSN. Move the look-up code to a
new file and introduce new setup methods.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c b/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
index 2836a19..b9a1ead 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_ctrl.c
@@ -380,7 +380,7 @@
 	int rc;
 
 
-	ctrl = controlif_setup(NULL, 4250);
+	ctrl = bsc_controlif_setup(NULL, 4250);
 	if (!ctrl) {
 		fprintf(stderr, "Failed to initialize the control interface. Exiting.\n");
 		return NULL;