add OpenSSL support for symmetric key encryption

There are a few other things I'd like to do before this is merged.

1. rename CRYPT_MODE_INTERNAL etc to CRYPT_ENGINE_INTERNAL 
2. adjust ordering of crypto algorithms in SSH2 based on available extensions
3. do a test of RC4 too for CRYPT_MODE_STREAM 
4. test both internal modes?

To elaborate on #2...  OpenSSL cannot be used with arcfour256, for example. That uses a 256-bit key whereas OpenSSL only supports RC4 up to 128-bit keys.