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/hostcalls_impl/fs.rs

@@ -5,7 +5,7 @@ use crate::ctx::WasiCtx;
 use crate::fdentry::FdEntry;
 use crate::helpers::systemtime_to_timestamp;
 use crate::hostcalls_impl::{fd_filestat_set_times_impl, PathGet};
-use crate::sys::fdentry_impl::determine_type_rights;
+use crate::sys::fdentry_impl::{determine_type_rights, OsFile};
 use crate::sys::host_impl;
 use crate::sys::hostcalls_impl::fs_helpers::PathGetExt;
 use crate::{host, Error, Result};
@@ -167,7 +167,7 @@ pub(crate) fn path_open(
 }
 
 pub(crate) fn fd_readdir(
-    fd: &File,
+    fd: &mut OsFile,
     host_buf: &mut [u8],
     cookie: host::__wasi_dircookie_t,
 ) -> Result<usize> {