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/poll.rs

@@ -35,7 +35,7 @@ impl PollFd {
     }
 }
 
-pub fn poll(fds: &mut [PollFd], timeout: i32) -> Result<usize> {
+pub fn poll(fds: &mut [PollFd], timeout: libc::c_int) -> Result<usize> {
     Errno::from_result(unsafe {
         libc::poll(
             fds.as_mut_ptr() as *mut libc::pollfd,