ls: issue error message on removed directory

ls: issue error message on removed directory

If the current directory has been removed, then "ls" confusingly
produced no output and no error message, indistinguishable from
running on an empty directory.

* src/ls.c (print_dir): Report ENOENT on GNU/Linux if readdir
finds no directory entries at all, not even "." or "..",
and a recheck with the getdents syscall returns ENOENT.
We recheck with getdents() as POSIX states that
"The directory entries for dot and dot-dot are optional".
* tests/ls/removed-directory.sh: New file.
* tests/local.mk (all_tests): Add new test.
* NEWS: Mention the change in behavior.
Reported by Owen Thomas.