[BUG] fchmodat with AT_SYMLINK_NOFOLLOW is very inconsistent

I'm seeing three different behaviors from `fchmodat` on a symlink when using `AT_SYMLINK_NOFOLLOW`:

1. The flag is completely ignored because the kernel doesn't implement it; the mode of the referent is changed (this was fixed in early 2015, so this is the case prior to API 23 devices).
2. Symlink permissions are not supported and `ENOSUP` is set
3. Symlink permissions are supported and the symlink mode is set appropriately

Both 2 and 3 provide a conformant implementation of `std::filesystem::permissions(symlink_path, p perms, std::filesystem::perm_options::nofollow)`, but 1 does not. The permissions of the symlink need to either be set or an error needs to be returned.