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

build.rs

@@ -154,26 +154,11 @@ fn avoid_keywords(name: &str) -> &str {
 }
 
 cfg_if::cfg_if! {
-    if #[cfg(linux)] {
-        /// Ignore tests that aren't supported yet.
-        fn ignore(_testsuite: &str, _name: &str) -> bool {
-            if testsuite == "misc_testsuite" {
-                match name {
-                    "path_rename_trailing_slashes" => true,
-                    "path_symlink_trailing_slashes" => true,
-                    "remove_directory_trailing_slashes" => true,
-                    _ => false,
-                }
-            } else {
-                unreachable!()
-            }
-        }
-    } else if #[cfg(not(any(linux, windows)))] {
+    if #[cfg(not(windows))] {
         /// Ignore tests that aren't supported yet.
         fn ignore(testsuite: &str, name: &str) -> bool {
             if testsuite == "misc_testsuite" {
                 match name {
-                    "fd_readdir" => true,
                     "path_rename_trailing_slashes" => true,
                     "path_symlink_trailing_slashes" => true,
                     "remove_directory_trailing_slashes" => true,