privops: ignore signals in helper

If the whole process group receives a signal (e.g. CTRL-C in terminal),
the helper process needs to keep running until it gets the QUIT request,
so the system drivers can still use it in their finalisation, e.g. to
cancel remaining slew.

privops.c

@@ -520,6 +520,9 @@ PRV_StartHelper(void)
         close(fd);
     }
 
+    /* ignore signals, the process will exit on OP_QUIT request */
+    UTI_SetQuitSignalsHandler(SIG_IGN);
+
     helper_main(sock_pair[1]);
 
   } else {