The log server now advertises a subcommands flag if it supports
logging subcommands (e.g. commands run from a sudo-spawned program
like a shell). The client should only log additional commands
during a session if this flag is set in the ServerHello message.
This makes it easier to detect a plaintext client sending to a
TLS port. Without this, the TLS server will be silent as it
waits for the client to initiate the TLS connection.
