Pass password to OpenSSL engine by user interface

Recent OpenSSL uses user interface abstraction to negotiate access to
private keys in the cryprographical engines. An OpenSSL application is
expected to implement the user interface. Otherwise a default one
provided by OpenSSL (interactive standard I/O) will be used and the
aplication will have no way how to pass a password to the engine.

Longer-desc: http://curl.haxx.se/mail/lib-2013-08/0265.html