Root: should we have Root::open_subpath_follow?

[cyphar]

At the moment the only API we have which controls follow-trailing-symlink behaviour with O_NOFOLLOW is Root::open_subpath. Maybe we should just have Root::open_subpath_follow and Root::open_subpath?

[cyphar]

#271 is quite related -- if Root::resolve_nofollow is fine then Root::open_subpath have default-follow behaviour and being opt-out with O_NOFOLLOW is probably fine. FWIW, this was my thinking when I was putting together things for 0.2...