[PATCH 0 of 1] allow to use engine keyform for server private key

[Tatiana Kondakova]

Hello.
I'm a cryptography library developer (http://www.cryptopro.ru/).
I want to make our server-side TLS worked with nginx, and we have engine for openssl, which successfully works with openssl utilities. But for security reasons we can not export the private key to a file, so our engine needs something like keyform ENGINE option.
This option makes possible to use nginx with our library, with PKCS#11 tokens and with any other engine, which does not support private keys export.