Fix fd_readdir on BSD-style nixes (#81)

* Fix fd_readdir on BSD-style nixes

The fix was tested on Darwin-XNU and FreeBSD. The change introduces
thread-safe cache of (RawFd, *mut libc::DIR) pairs so that
libc::fdopendir syscall is called only once when invoking fd_readdir
for the first time, and then the pointer to the directory stream,
*mut libc::DIR, is reused until the matching raw file descriptor
is closed.

This fix allows then correct use (and matching to the implementation
on Linux kernels) of libc::seekdir and libc::rewinddir to seek through
and rewind the existing directory stream, *mut libc::DIR, which
otherwise seems to be reset/invalidated every time libc::fdopendir
is called (unlike on Linux, where this behaviour is not observed).

* Store dir stream as part of the FdEntry's Descriptor

* Move bsd specifics into separate module

* Add todo comments and fix formatting

* Refactor int conversions

* Emphasise in debug logs that we're looking at fd_readdir entry

* Change visibility of FdEntry and related to public-private

* Rewrite creating DirStream for the first time

src/sys/windows/fdentry_impl.rs

@@ -2,12 +2,42 @@ use crate::fdentry::Descriptor;
 use crate::{host, Error, Result};
 use std::fs::File;
 use std::io;
+use std::ops::{Deref, DerefMut};
 use std::os::windows::prelude::{AsRawHandle, FromRawHandle, RawHandle};
 
+#[derive(Debug)]
+pub(crate) struct OsFile(File);
+
+impl From<File> for OsFile {
+    fn from(file: File) -> Self {
+        Self(file)
+    }
+}
+
+impl AsRawHandle for OsFile {
+    fn as_raw_handle(&self) -> RawHandle {
+        self.0.as_raw_handle()
+    }
+}
+
+impl Deref for OsFile {
+    type Target = File;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for OsFile {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
 impl AsRawHandle for Descriptor {
     fn as_raw_handle(&self) -> RawHandle {
         match self {
-            Descriptor::File(f) => f.as_raw_handle(),
+            Descriptor::OsFile(file) => file.as_raw_handle(),
             Descriptor::Stdin => io::stdin().as_raw_handle(),
             Descriptor::Stdout => io::stdout().as_raw_handle(),
             Descriptor::Stderr => io::stderr().as_raw_handle(),