ossl: Add support for ETA mode

[kd]

Summary

With the support for AES-CBC we can now handle ETA requests.

[jhb]

sys/crypto/openssl/ossl.c

176

I don't think ETA makes sense for CHACHA20. I would perhaps require sc->has_aes as the only likely candidates for ETA use (AES-CBC, AES-CTR, and AES-XTS) will all need has_aes

[kd]

Don't allow for creating ETA sessions witch CHACHA20 used as a cipher.

[kd]

sys/crypto/openssl/ossl.c

176

Good point. Since we have AEAD of CHACH20+POLY1305 you're right that it doesn't make sense to try to use it for ETA.
I inverted part of this logic instead, checking just for sc->has_aes here would still allow someone to create an ETA session with CHACHA20 as the cipher.