Add yanix crate and replace nix with yanix in wasi-common (#649)

* Add yanix crate

This commit adds `yanix` crate as a Unix dependency for `wasi-common`.
`yanix` stands for Yet Another Nix crate and is exactly what the name
suggests: a crate in the spirit of the `nix` crate, but which takes a different
approach, using lower-level interfaces with less abstraction, so that it fits
better with its main use case, implementation of WASI syscalls.

* Replace nix with yanix crate

Having introduced `yanix` crate as an in-house replacement for the
`nix` crate, this commit makes the necessary changes to `wasi-common`
to depend _only_ on `yanix` crate.

* Address review comments

* make `fd_dup` unsafe
* rename `get_fd` to `get_fd_flags`, etc.
* reuse `io::Error::last_os_error()` to get the last errno value

* Address more comments

* make all `fcntl` fns unsafe
* adjust `wasi-common` impl appropriately

* Make all fns operating on RawFd unsafe

* Fix linux build

* Address more comments

crates/wasi-common/src/sys/mod.rs

@@ -8,7 +8,7 @@ cfg_if! {
         pub use self::unix::preopen_dir;
 
         pub(crate) fn errno_from_host(err: i32) -> wasi::__wasi_errno_t {
-            host_impl::errno_from_nix(nix::errno::from_i32(err)).as_wasi_errno()
+            host_impl::errno_from_nix(yanix::Errno::from_i32(err)).as_wasi_errno()
         }
     } else if #[cfg(windows)] {
         mod windows;