a copy-paste bug in UnixSignalWatcherPrivate::watchForSignal

[zmiiGorinich]

Thanks for a great piece of code!
There is only couple of problems:

1. You seem to forget to copy the sa_flags=0 initialization from the Qt example.
   In my case it leads to not restoring the handler after the first call, time to time.
2. I am not sure why did Qt people use sigemptyset in the original example.
   Possibly, they wanted to add the signals later with sigaddset, but forgot about it.
   Anyway, now your handler is not protected from any signal to arrive during its
   execution. I doubt it's the desired effect.