cryptosoft: Fully support per-operation keys for auth algorithms.

Only pre-allocate auth contexts when a session-wide key is provided or
for sessions without keys. For sessions with per-operation keys,
always initialize the on-stack context directly rather than
initializing the session context in swcr_authprepare (now removed) and
then copying that session context into the on-stack context.

This approach permits parallel auth operations without needing a
serializing lock. In addition, the previous code assumed that auth
sessions always provided an initial key unlike cipher sessions which
assume either an initial key or per-op keys.

While here, fix the Blake2 auth transforms to function like other auth
transforms where Setkey is invoked after Init rather than before.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33316