Implement unlinkat - Update error handling to use unwrap_err

The previous patch failed testing on two targets that returned EPERM
as the Sys error instead of EISDIR. This patch changes from using the
should_panic attribute to using the unwrap_err and matching against
the result.

CHANGELOG.md

@@ -77,7 +77,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Add `sched_yield`
   ([#1050](https://github.com/nix-rust/nix/pull/1050))
 - Add `unlinkat`
-  ([#TBD](https://github.com/nix-rust/nix/pull/1058))
+  ([#1058](https://github.com/nix-rust/nix/pull/1058))
 
 ### Changed
 - `PollFd` event flags renamed to `PollFlags` ([#1024](https://github.com/nix-rust/nix/pull/1024/))

test/test_unistd.rs

@@ -5,12 +5,12 @@ use nix::sys::signal::{SaFlags, SigAction, SigHandler, SigSet, Signal, sigaction
 use nix::sys::wait::*;
 use nix::sys::stat::{self, Mode, SFlag};
 use nix::errno::Errno;
+use nix::Error;
 use std::{env, iter};
 use std::ffi::CString;
 use std::fs::{self, DirBuilder, File};
 use std::io::Write;
 use std::os::unix::prelude::*;
-use std::panic;
 use tempfile::{self, tempfile};
 use libc::{self, _exit, off_t};
 
@@ -602,7 +602,6 @@ fn test_symlinkat() {
 
 
 #[test]
-#[should_panic (expected="`Err` value: Sys(EISDIR)")]
 fn test_unlinkat_dir_noremovedir() {
     let tempdir = tempfile::tempdir().unwrap();
     let dirname = "foo_dir";
@@ -615,7 +614,12 @@ fn test_unlinkat_dir_noremovedir() {
     let dirfd = fcntl::open(tempdir.path(), fcntl::OFlag::empty(), stat::Mode::empty()).unwrap();
 
     // Attempt unlink dir at relative path without proper flag
-    unlinkat(Some(dirfd), dirname, UnlinkatFlags::NoRemoveDir).unwrap();
+    let err_result = unlinkat(Some(dirfd), dirname, UnlinkatFlags::NoRemoveDir).unwrap_err();
+    match err_result {
+        Error::Sys(Errno::EISDIR) => println!("{}", err_result),
+        Error::Sys(Errno::EPERM) => println!("{}", err_result),
+        _ => panic!("Unlinkat did not fail as expected: {}", err_result),
+    }  
  }
 
 #[test]