Instead, we store the data we need from the return value of
readdir in an object of the new type struct readdir_result.
This type is independent of the layout of struct dirent.
Previously, application code had to set up the d_namlen member if
the target supported it, involving conditional compilation. After
this change, glob will use the length of the string in d_name instead
of d_namlen to determine the file name length. All glibc targets
provide the d_type and d_ino members, and setting them as needed for
gl_readdir is straightforward.
Changing the behavior with regards to d_ino is left to a future
cleanup.
