Make `ls' a lot more efficient on systems (e.g., linux-2.4.*)

Make `ls' a lot more efficient on systems (e.g., linux-2.4.*)
that save file type information in directory entries.

[enum filetype] (unknown):
Add members (as yet unused):
(HAVE_STRUCT_DIRENT_D_TYPE): Define.
(format_needs_type): New global.
(main): Set it.
(print_dir): Set `type' from directory entry, if possible.
(gobble_file): Add a parameter, TYPE.
Stat the file only if its type is unknown and we need the type.
Patch from Ulrich Drepper.