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/hostcalls/fs.rs

@@ -43,7 +43,7 @@ hostcalls! {
     ) -> wasm32::__wasi_errno_t;
 
     pub fn fd_seek(
-        wasi_ctx: &WasiCtx,
+        wasi_ctx: &mut WasiCtx,
         memory: &mut [u8],
         fd: wasm32::__wasi_fd_t,
         offset: wasm32::__wasi_filedelta_t,
@@ -52,7 +52,7 @@ hostcalls! {
     ) -> wasm32::__wasi_errno_t;
 
     pub fn fd_tell(
-        wasi_ctx: &WasiCtx,
+        wasi_ctx: &mut WasiCtx,
         memory: &mut [u8],
         fd: wasm32::__wasi_fd_t,
         newoffset: wasm32::uintptr_t,
@@ -139,7 +139,7 @@ hostcalls! {
     ) -> wasm32::__wasi_errno_t;
 
     pub fn fd_readdir(
-        wasi_ctx: &WasiCtx,
+        wasi_ctx: &mut WasiCtx,
         memory: &mut [u8],
         fd: wasm32::__wasi_fd_t,
         buf: wasm32::uintptr_t,