Support log KeepMany

[Sytten]

Right now you have only KeepAll or KeepOne, neither are really good IMO.
Usually you want a balance where you keep the last X files, either by restarts or (ideally) by day.

So the new enum could look like:

enum LogRotation {
  KeepOne
  KeepAll
  KeepMany(usize)
}

[FabianLars]

#677