Issue #1622 port du to windows (#1788)

* Issue #1622 port `du` to windows

* Attempt to support Rust 1.32

Old version was getting "attributes are not yet allowed on `if`
expressions" on Rust 1.32

* Less #[cfg]

* Less duplicate code.

I need the return and the semicolon after if otherwise the second #[cfg]
leads to unexpected token complilation error

* More accurate size on disk calculations for windows

* Expect the same output on windows as with WSL

* Better matches output from du on WSL

* In the absence of feedback I'm disabling these tests on Windows.
They require `ln`. Windows does not ship with this utility.

* Use the coreutils version of `ln` to test `du`

`fn ccmd` is courtesy of @Artoria2e5

* Look up inodes (file ids) on Windows

* One more #[cfg(windows)] to prevent unreachable statement warning on linux

Cargo.toml

@@ -43,6 +43,7 @@ feat_common_core = [
     "df",
     "dircolors",
     "dirname",
+    "du",
     "echo",
     "env",
     "expand",
@@ -149,7 +150,6 @@ feat_require_unix = [
     "chmod",
     "chown",
     "chroot",
-    "du",
     "groups",
     "hostid",
     "id",