We had the password callback for ENGINEs pretty much wrong. And

passwords that were given to the key loading functions were completely
ignored, at least in the ncipher code, and then we made the assumption
that the callback wanted a prompt as user argument.

All that is now changed, and the application author is forced to give
a callback function of type pem_callback_cb and possibly an argument
for it, just as for all other functions that want to generate password
prompting.

NOTE: this change creates binary and source incompatibilities with
previous versions of OpenSSL [engine].  It's worth it this time, to
get it right (or at least better and with a chance that it'll work).