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Various different BSD systems use a different errno from `E_LOOP` (as
defined by POSIX and used on Linux) to indicate that a file is a symlink
when you try to `open()` it with `O_NOFOLLOW`.

Fix the code which detects this. This is a follow-up to #1302.
Signed-off-by: Philip Withnall <withnall@endlessm.com>