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
Reviewed by: kib, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8586