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/fcntl.rs

@@ -0,0 +1,33 @@
+use crate::{
+    file::{FdFlag, OFlag},
+    Errno, Result,
+};
+use std::os::unix::prelude::*;
+
+pub unsafe fn dup_fd(fd: RawFd, close_on_exec: bool) -> Result<RawFd> {
+    // Both fcntl commands expect a RawFd arg which will specify
+    // the minimum duplicated RawFd number. In our case, I don't
+    // think we have to worry about this that much, so passing in
+    // the RawFd descriptor we want duplicated
+    Errno::from_result(if close_on_exec {
+        libc::fcntl(fd, libc::F_DUPFD_CLOEXEC, fd)
+    } else {
+        libc::fcntl(fd, libc::F_DUPFD, fd)
+    })
+}
+
+pub unsafe fn get_fd_flags(fd: RawFd) -> Result<FdFlag> {
+    Errno::from_result(libc::fcntl(fd, libc::F_GETFD)).map(FdFlag::from_bits_truncate)
+}
+
+pub unsafe fn set_fd_flags(fd: RawFd, flags: FdFlag) -> Result<()> {
+    Errno::from_success_code(libc::fcntl(fd, libc::F_SETFD, flags.bits()))
+}
+
+pub unsafe fn get_status_flags(fd: RawFd) -> Result<OFlag> {
+    Errno::from_result(libc::fcntl(fd, libc::F_GETFL)).map(OFlag::from_bits_truncate)
+}
+
+pub unsafe fn set_status_flags(fd: RawFd, flags: OFlag) -> Result<()> {
+    Errno::from_success_code(libc::fcntl(fd, libc::F_SETFL, flags.bits()))
+}