unlinkat: handle ignoring of ".." on Darwin 14 * lib/unlinkat.c: unlinkat() has the same bug as unlink() on Mac OS X 10.10, where it ignores paths with a trailing "..", so handle in the same manner. * m4/unlinkat.m4: Comment on this Darwin issue. * doc/posix-functions/unlink.texi: Update the latest version where the issue was seen. * doc/posix-functions/unlinkat.texi: Mention this issue. Fixes a test failure in test-unlinkat.c.