Misc yanix fixes (#715)

* 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.

crates/wasi-common/yanix/src/clock.rs

@@ -27,3 +27,11 @@ pub fn clock_getres(clock_id: ClockId) -> Result<libc::timespec> {
     })?;
     Ok(unsafe { timespec.assume_init() })
 }
+
+pub fn clock_gettime(clock_id: ClockId) -> Result<libc::timespec> {
+    let mut timespec = MaybeUninit::<libc::timespec>::uninit();
+    Errno::from_success_code(unsafe {
+        libc::clock_gettime(clock_id.as_raw(), timespec.as_mut_ptr())
+    })?;
+    Ok(unsafe { timespec.assume_init() })
+}