sinksmtp: when logging SMTP, detect and block do-nothing clients

Our definition of clients that do something is ones that successfully
EHLO/HELO. A yakker gets a 'stall all' set of rules and their actions
are not logged. Since the purpose is to keep yakkers out of SMTP logs,
we don't squelch yakkers if SMTP logging is off.
sinksmtp: make the 'suppress do-nothing clients' behavior optional

New command line argument -noyakkers.