support seteuid usecases

Currently the code uses Uid::current internally. This makes it impossible for the root user to connect to other user's dbus instances by means of seteuid.

To support this, switch to Uid::effective.

Possible issues:

I am not aware of any normal cases where the euid would be set without the library users explicitly setting it. And if that is happening, it seems unlikely that zbus's current behavior is usefull. So I don't expect this will break any existing use-cases.