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/src/old/snapshot_0/host.rs

@@ -63,12 +63,12 @@ impl Dirent {
 
         let sys_dirent = raw.as_mut_ptr() as *mut __wasi_dirent_t;
         unsafe {
-            *sys_dirent = __wasi_dirent_t {
+            sys_dirent.write_unaligned(__wasi_dirent_t {
                 d_namlen: namlen.try_into()?,
                 d_ino: self.ino,
                 d_next: self.cookie,
                 d_type: self.ftype.to_wasi(),
-            };
+            });
         }
 
         let sys_name = unsafe { sys_dirent.offset(1) as *mut u8 };

crates/wasi-common/src/old/snapshot_0/sys/unix/hostcalls_impl/misc.rs

@@ -3,7 +3,7 @@
 use crate::old::snapshot_0::hostcalls_impl::{ClockEventData, FdEventData};
 use crate::old::snapshot_0::sys::host_impl;
 use crate::old::snapshot_0::{wasi, Error, Result};
-use yanix::clock::{clock_getres, ClockId};
+use yanix::clock::{clock_getres, clock_gettime, ClockId};
 
 fn wasi_clock_id_to_unix(clock_id: wasi::__wasi_clockid_t) -> Result<ClockId> {
     // convert the supported clocks to libc types, or return EINVAL
@@ -39,7 +39,7 @@ pub(crate) fn clock_res_get(clock_id: wasi::__wasi_clockid_t) -> Result<wasi::__
 
 pub(crate) fn clock_time_get(clock_id: wasi::__wasi_clockid_t) -> Result<wasi::__wasi_timestamp_t> {
     let clock_id = wasi_clock_id_to_unix(clock_id)?;
-    let timespec = clock_getres(clock_id)?;
+    let timespec = clock_gettime(clock_id)?;
 
     // convert to nanoseconds, returning EOVERFLOW in case of overflow; this is freelancing a bit
     // from the spec but seems like it'll be an unusual situation to hit