ln: fix replacing symbolic links whose targets can't exist

* src/ln.c (errno_nonexisting): A new function to determine if
the errno implies that a file doesn't or can't (currently) exist.
(target_directory_operand): Use the new function to expand the
set of errors we handle.
* tests/ln/sf-1.sh: Add test cases for the newly handled errors.
* THANKS.in: Mention the reporter.
* NEWS: Mention the bug fix.

tests/ln/sf-1.sh

@@ -20,12 +20,27 @@
 print_ver_ ln
 
 echo foo > a || framework_failure_
-ln -s . b || framework_failure_
 
+# Check that a target directory of '.' is supported
+# and that indirectly specifying the same target and link name
+# through that is detected.
+ln -s . b || framework_failure_
 ln -sf a b > err 2>&1 && fail=1
 case $(cat err) in
   *'are the same file') ;;
   *) fail=1 ;;
 esac
 
+# Ensure we replace symlinks that don't or can't link to an existing target.
+# coreutils-8.22 would fail to replace {ENOTDIR,ELOOP,ENAMETOOLONG}_link below.
+name_max_plus1=$(expr $(stat -f -c %l .) + 1)
+test $name_max_plus1 -gt 1 || skip_ 'Error determining NAME_MAX'
+long_name=$(printf '%0*d' $name_max_plus1 0)
+for f in '' f; do
+  ln -s$f missing ENOENT_link || fail=1
+  ln -s$f a/b ENOTDIR_link || fail=1
+  ln -s$f ELOOP_link ELOOP_link || fail=1
+  ln -s$f "$long_name" ENAMETOOLONG_link || fail=1
+done
+
 Exit $fail