Use Unix.getuid instead of getlogin

[SquidDev]

This ensures that processes launched via sudo -u can access the bus.

While this does mean we're using Unix instead of Lwt_unix, in practice I don't think this matters as getuid is incredibly cheap.

[meadofpoetry]

The only thing that concerns me here is that this wont work on windows. But tbh I don't know if it works on windows, nor that anybody uses it there, so let's merge it that way.

[meadofpoetry]

Thanks for contribution