open: detect FreeBSD bug

open: detect FreeBSD bug

open("link-to-file/", O_RDONLY) mistakenly succeeds.  The previous
patch was enough to fix utimens when no fd is involved, but this is
necessary for futimens to pass.

* m4/open.m4 (gl_FUNC_OPEN): Also detect FreeBSD bug with slash on
symlink.
* doc/posix-functions/open.texi (open): Document the bug.
* doc/posix-functions/utimes.texi (utimes): Likewise.
* tests/test-open.h (test_open): Add parameters, and test symlink
handling.
* tests/test-open.c (main): Adjust caller.
* tests/test-fcntl-safer.c (main): Likewise.
* modules/open-tests (Depends-on): Add stdbool, symlink.
* modules/fcntl-safer-tests (Depends-on): Likewise.
* tests/test-openat.c (main): Add test-open tests.

Signed-off-by: Eric Blake <ebb9@byu.net>