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/libbsc/bts_ipaccess_nanobts.c b/openbsc/src/libbsc/bts_ipaccess_nanobts.c
index 097a889..13bed22 100644
--- a/openbsc/src/libbsc/bts_ipaccess_nanobts.c
+++ b/openbsc/src/libbsc/bts_ipaccess_nanobts.c
@@ -443,7 +443,7 @@
 	gsm_btsmodel_set_feature(&model_nanobts, BTS_FEAT_GPRS);
 	gsm_btsmodel_set_feature(&model_nanobts, BTS_FEAT_EGPRS);
 
-	register_signal_handler(SS_NM, nm_sig_cb, NULL);
+	osmo_signal_register_handler(SS_NM, nm_sig_cb, NULL);
 
 	return gsm_bts_model_register(&model_nanobts);
 }