Handling of negative tv_secs values

[sunfishcode]

Linux uses unsigned types for stat fields like st_mtime which are actually signed. Values with the most significant bit indicate negative time offsets from the epoch, which is to say, dates before 1970. Currently rustix just re-exports the kernel's stat struct, so it inherits this problem of using unsigned types for things that hold signed data.

rustix should change to using its own type for stat. That's also a chance to make st_mode be a Mode, st_uid be a Uid, and so on, and maybe even have things like st_mtime/st_mtime_nsec be a Timespec.

While here, macOS has a bug with negative timestamps, details here, and rustix should consider applying a similar fix.