insta-panic in softclock() -> acctwatch()

I've finally managed to catch one of the panics of a 2006.01.25 kernel
in action, while working on the console.  Unfortunately, not with a
debugging kernel, but the function backtrace was:

    panic()
    sleepq_add()
    cv_wait_unlock()
    cv_wait()
    _sx_xlock()
    acctwatch()
    softclock()

This went away after I disabled process accounting in `rc.conf', but it
may help someone more experienced with process accounting to find a
better fix :)

- Giorgos