Beginnings of EVP cipher overhaul. This should eventually

Beginnings of EVP cipher overhaul. This should eventually
enhance and tidy up the EVP interface.

This patch adds initial support for variable length ciphers
and changes S/MIME code to use this.

Some other library functions need modifying to support use
of modified cipher parameters.

Also need to change all the cipher functions that should
return error codes, but currenly don't.

And of course it needs extensive testing...