MFC r309026: open(2): Clarify non-POSIX error when opening a symlink with
O_NOFOLLOW.

We return [EMLINK] instead of [ELOOP] when trying to open a symlink with
O_NOFOLLOW, so that the original case of [ELOOP] can be distinguished. Code
like cmp -h and xz takes advantage of this.

PR: 214633