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/sys/linux/fadvise.rs

@@ -1,4 +1,4 @@
-use crate::{Errno, Result};
+use crate::{Error, Result};
 use std::os::unix::prelude::*;
 
 #[derive(Debug, Copy, Clone)]
@@ -18,5 +18,5 @@ pub unsafe fn posix_fadvise(
     len: libc::off_t,
     advice: PosixFadviseAdvice,
 ) -> Result<()> {
-    Errno::from_success_code(libc::posix_fadvise(fd, offset, len, advice as libc::c_int))
+    Error::from_success_code(libc::posix_fadvise(fd, offset, len, advice as libc::c_int))
 }