Auto merge of #31360 - pitdicker:fs_tests_cleanup, r=alexcrichton

- use `symlink_file` and `symlink_dir` instead of the old `soft_link`
- create a junction instead of a directory symlink for testing recursive_rmdir (as it causes the
  same troubles, but can be created by users without `SeCreateSymbolicLinkPrivilege`)
- `remove_dir_all` was unable to remove directory symlinks and junctions
- only run tests that create symlinks if we have the right permissions.
- rename `Path2` to `Path`
- remove the global `#[allow(deprecated)]` and outdated comments
- After factoring out `create_junction()` from the test `directory_junctions_are_directories` and
  removing needlessly complex code, what I was left with was:
  ```
  #[test]
  #[cfg(windows)]
  fn directory_junctions_are_directories() {
      use sys::fs::create_junction;

      let tmpdir = tmpdir();

      let foo = tmpdir.join("foo");
      let bar = tmpdir.join("bar");

      fs::create_dir(&foo).unwrap();
      check!(create_junction(&foo, &bar));
      assert!(bar.metadata().unwrap().is_dir());
  }
  ```
  It test whether a junction is a directory instead of a reparse point. But it actually test the
  target of the junction (which is a directory if it exists) instead of the junction itself, which
  should always be a symlink. So this test is invalid, and I expect it only exists because the
  author was suprised by it. So I removed it.

Some things that do not yet work right:
- relative symlinks do not accept forward slashes
- the conversion of paths for `create_junction` is hacky
- `remove_dir_all` now messes with the internal data of `FileAttr` to be able to remove symlinks.
  We should add some method like `is_symlink_dir()` to it, so code outside the standard library
  can see the difference between file and directory symlinks too.

src/libstd/sys/unix/fs.rs

@@ -554,6 +554,18 @@ pub fn rmdir(p: &Path) -> io::Result<()> {
     Ok(())
 }
 
+pub fn remove_dir_all(path: &Path) -> io::Result<()> {
+    for child in try!(readdir(path)) {
+        let child = try!(child);
+        if try!(child.file_type()).is_dir() {
+            try!(remove_dir_all(&child.path()));
+        } else {
+            try!(unlink(&child.path()));
+        }
+    }
+    rmdir(path)
+}
+
 pub fn readlink(p: &Path) -> io::Result<PathBuf> {
     let c_path = try!(cstr(p));
     let p = c_path.as_ptr();