Cygwin: implement dirent.d_reclen

Cygwin: implement dirent.d_reclen

This change is in preparation of adding posix_getdents() from
the upcoming POSIX Base Specification Issue 8.

- Add d_reclen
- Add GLibC compatible test macros for dirent members
- Bump dirent version
- Set d_reclen to the fixed size of the dirent struct
  We can do that because the size is a multiple of 8, so it fits
  snugly in the buffer filled by posix_getdents and keep the
  alignement.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>