Various cleanups to cryptocheck.

• Rename 'blkcipher' to 'cipher'. Some of the ciphers being tested are stream ciphers.
• Rename 'authenc' to 'eta' as it is only testing ETA chained operations and not other combination modes.
• Add a notion of an OCF session and some helper routines to try to reduce duplicated code. This also uses a single session for both encrypt and decrypt operations during a single test.
• Add tests to ensure that AEAD algorithms fail decryption with EBADMSG when given a corrupted tag.
• Remove the transitional hack for COP_F_CIPHER_FIRST.
• Update block comment to mention plain hashes.

Reviewed by: cem
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D22940