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/osmo_bsc_main.c b/openbsc/src/osmo-bsc/osmo_bsc_main.c
index 84186a6..f701089 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_main.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_main.c
@@ -212,7 +212,7 @@
 	}
 	bsc_api_init(bsc_gsmnet, osmo_bsc_api());
 
-	bsc_gsmnet->ctrl = controlif_setup(bsc_gsmnet, 4249);
+	bsc_gsmnet->ctrl = bsc_controlif_setup(bsc_gsmnet, 4249);
 	if (!bsc_gsmnet) {
 		fprintf(stderr, "Failed to init the control interface. Exiting.\n");
 		exit(1);