ls -i inefficiency

[Eric Blake]

On platforms with a working dirent.d_ino, 'ls -i' is
wastefully calling [l]stat() to get the inode number
rather than using the d_ino obtained from the
readdir().

The code currently has format_needs_stat and
format_needs_type; it sounds like we also need
format_needs_inode with the fallback that on
platforms without d_ino, format_needs_inode
implies format_needs_stat.

-- 
Eric Blake