Switch getUnixTimestamp to cross platform code

[Dfte]

This PR switches the following line of code from the getUnixTime function:

return datetime.fromtimestamp(getUnixTime(int(str(self.fields[key]))))

To a cross platform independant oneliner:

return datetime(1601, 1, 1) + timedelta(microseconds=int(str(self.fields[key])) // 10)

It was mentionned as an issue on NetExec for people using it on Windows, which would be the same for people using Impacket on Windows:

Pennyw0rth/NetExec#827

And it mitigates it: