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/yanix/src/socket.rs

@@ -0,0 +1,31 @@
+use crate::{Errno, Result};
+use std::os::unix::prelude::*;
+
+#[derive(Debug, Clone, Copy)]
+#[repr(i32)]
+pub enum SockType {
+    Stream = libc::SOCK_STREAM,
+    Datagram = libc::SOCK_DGRAM,
+    SeqPacket = libc::SOCK_SEQPACKET,
+    Raw = libc::SOCK_RAW,
+    Rdm = libc::SOCK_RDM,
+}
+
+pub unsafe fn get_socket_type(fd: RawFd) -> Result<SockType> {
+    use std::mem::{self, MaybeUninit};
+    let mut buffer = MaybeUninit::<SockType>::zeroed().assume_init();
+    let mut out_len = mem::size_of::<SockType>() as libc::socklen_t;
+    Errno::from_success_code(libc::getsockopt(
+        fd,
+        libc::SOL_SOCKET,
+        libc::SO_TYPE,
+        &mut buffer as *mut SockType as *mut _,
+        &mut out_len,
+    ))?;
+    assert_eq!(
+        out_len as usize,
+        mem::size_of::<SockType>(),
+        "invalid SockType value"
+    );
+    Ok(buffer)
+}