Implement an improved version of the EVP AEAD API. The
EVP_AEAD_CTX_{open,seal} functions previously returned an ssize_t that was
overloaded to indicate success/failure, along with the number of bytes
written as output. This change adds an explicit *out_len argument which
is used to return the number of output bytes and the return value is now
an int that is purely used to identify success or failure.

This change effectively rides the last libcrypto crank (although I do not
expect there to be many users of the EVP AEAD API currently).

Thanks to Adam Langley for providing the improved code that this diff is
based on.

ok miod@