zero the frame pointer of the kernel stack of new threads to make ddb trace happier
zero the frame pointer of the kernel stack of new threads to make ddb trace
happier

ok dlg@ jsing@ kettenis@ mlarkin@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars