Safe FFI exercise: `dirent` layout is different on Mac OS

[mgeisler]

The safe FFI wrapper exercise uses the wrong layout on Mac OS.

[ericye16]

The man page for my system says:

>     The directory entry format is defined in the file ⟨sys/dirent.h⟩ and further in the file ⟨dirent.h⟩.  When the macro
     _DARWIN_FEATURE_64_BIT_INODE is not defined (see stat(2) for more information on this macro), the dirent structure is
     defined as:

     /*** Excerpt from <sys/dirent.h> ***/
     /*
      * The dirent structure defines the format of directory entries.
      *
      * A directory entry has a struct dirent at the front of it, containing its
      * inode number, the length of the entry, and the length of the name
      * contained in the entry.  These are followed by the name padded to a 4
      * byte boundary with null bytes.  All names are guaranteed null terminated.
      * The maximum length of a name in a directory is 255.
      */

     struct dirent { /* when _DARWIN_FEATURE_64_BIT_INODE is NOT defined */
             ino_t      d_ino;                /* file number of entry */
             __uint16_t d_reclen;             /* length of this record */
             __uint8_t  d_type;               /* file type, see below */
             __uint8_t  d_namlen;             /* length of string in d_name */
             char    d_name[255 + 1];   /* name must be no longer than this */
     };

     However, when the macro _DARWIN_FEATURE_64_BIT_INODE is defined, the dirent structure is defined as:

     /*
      * The dirent structure defines the format of directory entries.
      *
      * A directory entry has a struct dirent at the front of it, containing its
      * inode number, the length of the entry, and the length of the name
      * contained in the entry.  These are followed by the name padded to a 4
      * byte boundary with null bytes.  All names are guaranteed null terminated.
      * The maximum length of a name in a directory is 1023.
      */

     struct dirent { /* when _DARWIN_FEATURE_64_BIT_INODE is defined */
             ino_t      d_fileno;     /* file number of entry */
             __uint64_t d_seekoff;    /* seek offset (optional, used by servers) */
             __uint16_t d_reclen;     /* length of this record */
             __uint16_t d_namlen;     /* length of string in d_name */
             __uint8_t  d_type;       /* file type, see below */
             char    d_name[1024];    /* name must be no longer than this */
     };

[mgeisler]

Thanks @ericye16! @schultetwin1 was able to open a PR based on the man page above, so we should have this fixed now.