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/poll.rs

@@ -0,0 +1,47 @@
+use crate::{Errno, Result};
+use bitflags::bitflags;
+use std::{convert::TryInto, os::unix::prelude::*};
+
+bitflags! {
+    pub struct PollFlags: libc::c_short {
+        const POLLIN = libc::POLLIN;
+        const POLLPRI = libc::POLLPRI;
+        const POLLOUT = libc::POLLOUT;
+        const POLLRDNORM = libc::POLLRDNORM;
+        const POLLWRNORM = libc::POLLWRNORM;
+        const POLLRDBAND = libc::POLLRDBAND;
+        const POLLWRBAND = libc::POLLWRBAND;
+        const POLLERR = libc::POLLERR;
+        const POLLHUP = libc::POLLHUP;
+        const POLLNVAL = libc::POLLNVAL;
+    }
+}
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[repr(C)]
+pub struct PollFd(libc::pollfd);
+
+impl PollFd {
+    pub unsafe fn new(fd: RawFd, events: PollFlags) -> Self {
+        Self(libc::pollfd {
+            fd,
+            events: events.bits(),
+            revents: PollFlags::empty().bits(),
+        })
+    }
+
+    pub fn revents(self) -> Option<PollFlags> {
+        PollFlags::from_bits(self.0.revents)
+    }
+}
+
+pub fn poll(fds: &mut [PollFd], timeout: i32) -> Result<usize> {
+    Errno::from_result(unsafe {
+        libc::poll(
+            fds.as_mut_ptr() as *mut libc::pollfd,
+            fds.len() as libc::nfds_t,
+            timeout,
+        )
+    })
+    .and_then(|nready| nready.try_into().map_err(Into::into))
+}