crypto: Refactor software support for AEAD ciphers.

Extend struct enc_xform to add new members to handle auth operations
for AEAD ciphers. In particular, AEAD operations in cryptosoft no
longer use a struct auth_hash. Instead, the setkey and reinit methods
of struct enc_xform are responsible for initializing both the cipher
and auth state.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33196