Use AES256 and SHA256 by default, also for the meta-connections.

At the start of the decade, there were still distributions that shipped
with versions of OpenSSL that did not support these algorithms. By now
everyone should support them. The old defaults were Blowfish and SHA1,
both of which are not considered secure anymore.

The meta-protocol now always uses AES in OFB mode, but the key length
will adapt to the one specified by the Cipher option. The digest for the
meta-protocol is hardcoded to SHA256.