Add an AEAD EVP interface to libcrypto, along with AES-GCM AEAD
implementations. This largely pulls in Adam Langley's AEAD patches from
Chromium's OpenSSL.

ok miod@
Add a ChaCha20-Poly1305 AEAD EVP implementation to libcrypto, from Adam
Langley's Chromium OpenSSL patches.

ok miod@