src: use namespace prefix osmo_signal*

Summary of changes:

s/signal_cbfn/osmo_signal_cbfn/g
s/register_signal_handler/osmo_signal_register_handler/g
s/unregister_signal_handler/osmo_signal_unregister_handler/g
s/dispatch_signal/osmo_signal_dispatch/g
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_grace.c b/openbsc/src/osmo-bsc/osmo_bsc_grace.c
index f699cf3..c67984b 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_grace.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_grace.c
@@ -103,5 +103,5 @@
 
 static __attribute__((constructor)) void on_dso_load_grace(void)
 {
-	register_signal_handler(SS_RF, handle_rf_signal, NULL);
+	osmo_signal_register_handler(SS_RF, handle_rf_signal, NULL);
 }