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-nitb/bsc_hack.c b/openbsc/src/osmo-nitb/bsc_hack.c
index c78e3ec..61141fd 100644
--- a/openbsc/src/osmo-nitb/bsc_hack.c
+++ b/openbsc/src/osmo-nitb/bsc_hack.c
@@ -284,7 +284,7 @@
 #endif
 	bsc_api_init(bsc_gsmnet, msc_bsc_api());
 
-	bsc_gsmnet->ctrl = controlif_setup(bsc_gsmnet, 4249);
+	bsc_gsmnet->ctrl = bsc_controlif_setup(bsc_gsmnet, 4249);
 	if (!bsc_gsmnet->ctrl) {
 		printf("Failed to initialize control interface. Exiting.\n");
 		return -1;