unlink, remove: detect FreeBSD bug

unlink, remove: detect FreeBSD bug

unlink("link-to-file/") mistakenly removed "file".

* m4/unlink.m4 (gl_FUNC_UNLINK): Also detect FreeBSD bug with
slash on symlink.
* doc/posix-functions/unlink.texi (unlink): Document the bug.
* doc/posix-functions/remove.texi (remove): Likewise.
* tests/test-unlink.h (test_unlink): Enhance test.
* tests/test-remove.c (main): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit 24208770c8721323060207c9be83dd24400678d3)