logind: always use 64bit session IDs

logind: always use 64bit session IDs

it's a bit confusing that on 32bit systems we'd risk session IDs
overruns like this. Let's expose the same behaviour everywhere and stick
to 64bit ids.

Since we format the ids as strings anyway this doesn't really change
anything performance-wise, it just pushes out collisions by overrun to
basically never happen.