unix/modos: Implement ilistdir().

ilistdir() returns iterator which yields triples of (name, type, ino)
where ino is inode number for entry's data, type of entry (file/dir/etc.),
and name of file/dir. listdir() can be easily implemented in terms of this
iterator (which is otherwise more efficient in terms of memory use and may
save expensive call to stat() for each returned entry).

CPython has os.scandir() which also returns an iterator, but it yields
more complex objects of DirEntry type. scandir() can also be easily
implemented in terms of ilistdir().