* @val: NN value or SP array (preferred element first) to copy
* @len: true length of @val in bytes (excluding first element repetition)
* @repeat_first: whether to copy the first element of @val twice
+ *
* The last argument is used to construct Confirm options, where the preferred
* value and the preference list appear separately (RFC 4340, 6.3.1). Preference
* lists are kept such that the preferred entry is always first, so we only need