Fix humongous bug with fdopendir on mac

https://opensource.apple.com/source/Libc/Libc-1244.1.7/include/dirent.h.auto.html

ccall picks the wrong one, causes crashes and truncated d_name
entries. Wrapping the function in a small C shim, as well as
using the CAPI, fixes this.

hpath-posix/cbits/dirutils.c

@@ -1,7 +1,7 @@
 #include "dirutils.h"
+
 unsigned int
     __posixdir_d_type(struct dirent* d)
     {
       return(d -> d_type);
     }
-

hpath-posix/cbits/dirutils.h

@@ -7,7 +7,9 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+
 extern unsigned int
     __posixdir_d_type(struct dirent* d)
     ;
+
 #endif

hpath-posix/src/System/Posix/RawFilePath/Directory/Traversals.hs

@@ -10,6 +10,7 @@
 -- Traversal and read operations on directories.
 
 
+{-# LANGUAGE CApiFFI #-}
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE ForeignFunctionInterface #-}
 {-# LANGUAGE OverloadedStrings #-}
@@ -173,11 +174,15 @@ foreign import ccall unsafe "__hscore_d_name"
 foreign import ccall unsafe "__posixdir_d_type"
   c_type :: Ptr CDirent -> IO DirType
 
-foreign import ccall "realpath"
+foreign import capi "stdlib.h realpath"
   c_realpath :: CString -> CString -> IO CString
 
-foreign import ccall unsafe "fdopendir"
-  c_fdopendir :: Posix.Fd -> IO (Ptr ())
+-- Using normal 'ccall' here lead to memory bugs, crashes
+-- and corrupted d_name entries. It appears there are two fdopendirs:
+--   https://opensource.apple.com/source/Libc/Libc-1244.1.7/include/dirent.h.auto.html
+-- The capi call picks the correct one.
+foreign import capi unsafe "dirent.h fdopendir"
+  c_fdopendir :: Posix.Fd -> IO (Ptr CDir)
 
 ----------------------------------------------------------
 -- less dodgy but still lower-level