For now it seems that epoll is the preferred way to access Redox's
underlying polling interface. There are event schemes but until Redox's
syscalls are better integrated with Rustix it's probably not worth it
yet.

cc smol-rs/polling#176

Signed-off-by: John Nunley <dev@notgull.net>

Fix `proc_self_status()` to reset the directory cursor before iterating
through the directory entries when searching for bind mounts. This fixes
a failure when called more than once, due to the cursor being left at
the end.

Fixes #994.

Deprecate `Stat::st_mtime` etc., because they're unsigned and should be
signed, and introduce a `StatExt` extension trait which adds `mtime()`
etc. accessor functions which return the values in the appropriate
signed type.

This `StatExt` trait can go away next time we have a semver breaking
change, but for now this preserves compatibility.

On i686-apple-darwin, `time_t` is 32-bit, so it needs `fix_y2038` and
conversions for `Timespec` values.

Fixes #991.