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/poll.rs

@@ -1,4 +1,4 @@
-use crate::{Errno, Result};
+use crate::{Error, Result};
 use bitflags::bitflags;
 use std::{convert::TryInto, os::unix::prelude::*};
 
@@ -36,7 +36,7 @@ impl PollFd {
 }
 
 pub fn poll(fds: &mut [PollFd], timeout: libc::c_int) -> Result<usize> {
-    Errno::from_result(unsafe {
+    Error::from_result(unsafe {
         libc::poll(
             fds.as_mut_ptr() as *mut libc::pollfd,
             fds.len() as libc::nfds_t,