HEADS UP: panic behaviour changed

[Andrew Doran <ad%netbsd.org@localhost>]

I changed it so that the debugger is no longer automatically entered if the
system panics. Dropping to the debugger is especially inappropriate if the
victim is on call or using X at the time.

It will just dump and reboot now. To enter the debugger automatically set
ddb.onpanic=1 in /etc/sysctl.conf.

There's some scope for improvement here, for example waiting a few seconds
and prompting before reboot..

Andrew