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/abis_om2000.c b/openbsc/src/libbsc/abis_om2000.c
index 7743cd8..19d2042 100644
--- a/openbsc/src/libbsc/abis_om2000.c
+++ b/openbsc/src/libbsc/abis_om2000.c
@@ -753,7 +753,7 @@
 	nsd.new_state = &new_state;
 	nsd.om2k_mo = mo;
 
-	dispatch_signal(SS_NM, S_NM_STATECHG_ADM, &nsd);
+	osmo_signal_dispatch(SS_NM, S_NM_STATECHG_ADM, &nsd);
 
 	nm_state->availability = new_state.availability;
 }
@@ -794,7 +794,7 @@
 	nsd.new_state = nm_state;
 	nsd.om2k_mo = mo;
 
-	dispatch_signal(SS_NM, S_NM_STATECHG_OPER, &nsd);
+	osmo_signal_dispatch(SS_NM, S_NM_STATECHG_OPER, &nsd);
 }
 
 static int abis_om2k_sendmsg(struct gsm_bts *bts, struct msgb *msg)
@@ -1434,7 +1434,7 @@
 	nsd.obj = mo2obj(bts, &o2h->mo);
 	nsd.om2k_mo = &o2h->mo;
 	nsd.accordance_ind = acc;
-	dispatch_signal(SS_NM, S_NM_OM2K_CONF_RES, &nsd);
+	osmo_signal_dispatch(SS_NM, S_NM_OM2K_CONF_RES, &nsd);
 
 	return ret;
 }