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/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 6876939..5fdc5b7 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -105,7 +105,7 @@
 
 	switch (signal) {
 	case SIGINT:
-		dispatch_signal(SS_GLOBAL, S_GLOBAL_SHUTDOWN, NULL);
+		osmo_signal_dispatch(SS_GLOBAL, S_GLOBAL_SHUTDOWN, NULL);
 		sleep(1);
 		exit(0);
 		break;