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/sys/mod.rs

@@ -5,10 +5,10 @@ cfg_if! {
     if #[cfg(any(target_os = "linux",
                  target_os = "android"))] {
         mod linux;
-        pub(crate) use self::linux::*;
+        pub(crate) use linux::*;
     } else if #[cfg(target_os = "emscripten")] {
         mod emscripten;
-        pub(crate) use self::emscripten::*;
+        pub(crate) use emscripten::*;
     } else if #[cfg(any(target_os = "macos",
                         target_os = "ios",
                         target_os = "freebsd",
@@ -16,7 +16,7 @@ cfg_if! {
                         target_os = "openbsd",
                         target_os = "dragonfly"))] {
         mod bsd;
-        pub(crate) use self::bsd::*;
+        pub(crate) use bsd::*;
     } else {
         compile_error!("yanix doesn't compile for this platform yet");
     }