server/: Augment challenges to allow a payload.

  * Augment the external interface to take a pointer/length pair
    designating the payload.

  * Change the bulk-crypto interface to pass the sequence number in
    separately from the (repurposed) payload buffer.

  * Modify the `naclbox' challenge handling to hash the payload using
    Poly1305, rather than just producing some Salsa20 output.