Lower level of new "Logoff time <= logon time" log message

... from ERROR to WARNING, but log the human-readable logon time too.

This condition can easily happen if the system clock is corrected (e.g. a few seconds via NTP adjustment) during a quick connect/disconnect.

Don't call logoffstats() unless a user actually successfully logged-in. I don't think sysops actually want all the time bots sit at the logon prompt counted as "time online" in their stats anyway.