--encrypt-binlog and --encrypt-tmp-files used to mean
"encrypt XXX if encryption is available, otherwise don't encrypt",
now they mean "encrypt or fail with an error".
Instead of encrypt(src, dst, key, iv) that encrypts all
data in one go, now we have encrypt_init(key,iv),
encrypt_update(src,dst), and encrypt_finish(dst).
This also causes collateral changes in the internal my_crypt.cc
encryption functions and in the encryption service.
There are wrappers to provide the old all-at-once encryption
functionality. But binlog events are often written piecewise,
they'll need the new api.
