* Correctly handle possibly misaligned pointers in readdir
This reapplies #615, which was inadvertently reverted.
* Tidy up unneeded `self::` qualifiers.
* Make Dir's contents private.
Also remove the `unsafe` from `impl_iter`. With `Dir`'s field being
private, we can rely on the pointer being only what we've assigned to
it.
* Make `poll`'s timeout argument a `libc::c_int`.
This clarifies that there are no subsequent conversions before calling the
underlying libc API.
* Use clock_gettime instead of clock_getres to get the time.
* Mark FileType::from_raw as safe.
It handles unknown values, so it can be marked safe.
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