Add some more POSIX compatibility by adding a d_type field to the
dirent structure and defining corresponding macros so listdir_next
in the unix' port modos.c can use it, end result being uos.ilistdir
now reports the file type.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.