sd-varlink: use MSG_PEEK for protocol_upgrade connections

sd-varlink: use MSG_PEEK for protocol_upgrade connections

When there is a potential protocol upgrade we need to be careful that
we do not read beyond our json message as the custom protocol may be
anything. This was archived via a byte-by-byte read. This is of course
very inefficient. So this commit moves to use MSG_PEEK to find the
boundary of the json message instead. This makes the performance hit
a lot smaller.

Thanks to Lennart for suggesting this.