Duplicate definition of clockid

[maxbla]

Is there some reason that we have both nix::time::ClockId and nix::sys::timerfd::ClockId? It seems like nix::time::ClockId is more complete - defining more types of clocks for different operating systems. It looks to me like the timerfd implementation could be entirely replaced by the time one.

I would be happy to submit a PR removing nix::sys::timerfd::ClockId (if doing so makes sense). Obviously this would be a breaking change, so it may have to wait.

[maxbla]

It looks like nix::time::ClockId was added fairly recently in #1281. I suggest removing nix::sys::timerfd::ClockId when we're next allowed a breaking change.