crypto: Re-add encrypt/decrypt_multi hooks to enc_xform.

These callbacks allow multiple contiguous blocks to be manipulated in
a single call. Note that any trailing partial block for a stream
cipher must still be passed to encrypt/decrypt_last.

While here, document the setkey and reinit hooks and reorder the hooks
in 'struct enc_xform' to better reflect the life cycle.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33529