[abis] Add ACK signal and send it on SET NVATTR ACK

* Add the signal definition to signal.h
* Dispatch the signal from abis_nm.c
* Handle it in ipaccess-config.c and say we are done with work
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index 211f70e..9dedcaf 100644
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -2805,6 +2805,9 @@
 	case NM_MT_IPACC_GET_NVATTR_NACK:
 		dispatch_signal(SS_NM, S_NM_IPACC_NACK, &foh->msg_type);
 		break;
+	case NM_MT_IPACC_SET_NVATTR_ACK:
+		dispatch_signal(SS_NM, S_NM_IPACC_ACK, &foh->msg_type);
+		break;
 	default:
 		break;
 	}