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_msc.c b/openbsc/src/osmo-bsc/osmo_bsc_msc.c
index 5517d30..6033985 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_msc.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_msc.c
@@ -22,6 +22,7 @@
 
 #include <openbsc/bsc_nat.h>
 #include <openbsc/control_cmd.h>
+#include <openbsc/control_if.h>
 #include <openbsc/debug.h>
 #include <openbsc/gsm_data.h>
 #include <openbsc/ipaccess.h>
@@ -217,7 +218,7 @@
 		return;
 	}
 
-	ret = ctrl_cmd_handle(cmd, msc->network);
+	ret = bsc_ctrl_cmd_handle(cmd, msc->network);
 	if (ret != CTRL_CMD_HANDLED)
 		ctrl_cmd_send(&msc->msc_con->write_queue, cmd);
 	talloc_free(cmd);