Implement rfc6587 auto detection

[bazsi]

This is a pretty common misconfiguration that should be handled automatically.

Open questions were resolved this way:

• syslog(transport(tcp/tls)) will default to enable framing detection
• if you want to require framing transport(force-framed) will do that, but I doubt anyone needs that, so let's not document it.
• auto is only the name of the class, it's not user visible anymore.
• testcases are added