update OpenSSL calls to newer EVP interface
A new EVP interface was added in OpenSSL 1.0.0. It seems that it's usage should be more confortable and a bit safer when writing into size limitted buffers.
EVP_SignInit_ex -> EVP_DigestSignInit ...
OpenSSL 1.0.0 was released on 2010-03-29, but it seems unavailable on Debian and FreeBSD machines in Jenkins yet.