cygwin: unify reparse point checking code into single function

cygwin: unify reparse point checking code into single function

So far we had two functions checking the content of a reparse point,
readdir_check_reparse_point in fhandler_disk_file.cc for the sake of
readdir, and symlink_info::check_reparse_point for the sake of
generic path checking.

* Rename check_reparse_point_target helper to check_reparse_point_string
  and convert to static function.
* Create new check_reparse_point_target helper containing the core
  reparse point checking code
* Just call check_reparse_point_target from readdir_check_reparse_point
  and symlink_info::check_reparse_point and only perform the unique
  task in those functions.

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