wasi-common error cleanup: part 1, yanix (#1226)

* Reuse std::io::Error for raw *nix errno

This commit removes custom `yanix::Errno` and instead (as was
previously suggested) reuses `std::io::Error` to generate and wrap
raw *nix errno value.

* Update wasi-common to use new Yanix error type

This commit updates `wasi-common` to use new way of handling raw
OS error in `yanix`; i.e., via re-use of `std::io::Error` instead
of a custom `Errno` enum.

* Fix formatting

* Unwrap if io::Error created from raw OS error

This commit calls `unwrap` on `err` if that one was created via
`io::Error::last_os_error()`. It also refactors error matching
in several syscalls on the BSD platform (mainly).

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

@@ -1,10 +1,10 @@
 use crate::{
     file::FileType,
     sys::dir::{iter_impl, EntryImpl},
-    Errno, Result,
+    Result,
 };
 use std::os::unix::io::{AsRawFd, IntoRawFd, RawFd};
-use std::{ffi::CStr, ops::Deref, ptr};
+use std::{ffi::CStr, io, ops::Deref, ptr};
 
 pub use crate::sys::EntryExt;
 
@@ -25,7 +25,7 @@ impl Dir {
         if let Some(d) = ptr::NonNull::new(d) {
             Ok(Self(d))
         } else {
-            let e = Errno::last();
+            let e = io::Error::last_os_error();
             libc::close(fd);
             Err(e.into())
         }