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/libabis/input/dahdi.c b/openbsc/src/libabis/input/dahdi.c
index e02e141..9b85b7d 100644
--- a/openbsc/src/libabis/input/dahdi.c
+++ b/openbsc/src/libabis/input/dahdi.c
@@ -83,11 +83,11 @@
 	switch (evt) {
 	case DAHDI_EVENT_ALARM:
 		/* we should notify the code that the line is gone */
-		dispatch_signal(SS_INPUT, S_INP_LINE_ALARM, &isd);
+		osmo_signal_dispatch(SS_INPUT, S_INP_LINE_ALARM, &isd);
 		break;
 	case DAHDI_EVENT_NOALARM:
 		/* alarm has gone, we should re-start the SABM requests */
-		dispatch_signal(SS_INPUT, S_INP_LINE_NOALARM, &isd);
+		osmo_signal_dispatch(SS_INPUT, S_INP_LINE_NOALARM, &isd);
 		break;
 	}
 }