logind: don't make idle action timer accuracy more coarse than timeout

logind: don't make idle action timer accuracy more coarse than timeout

If we allow the timer accuracy to grow larger then the timeout itself
things are very confusing, because people might set a 1s time-out and we
turn that into 30s.

Hence, let's just cut off the 30s accuracy to the time-out itself, so
that we stay close to what users configured.