pidctrl_daemon() implements a variation on the classical, discrete PID

pidctrl_daemon() implements a variation on the classical, discrete PID
controller that tries to handle early invocations of the controller,
in other words, invocations before the expected end of the interval.
However, there were some calculation errors in this early invocation
case.  Notably, if an early invocation occurred while the error was
negative, the derivative term was off by a large amount.  One visible
effect of this error was that processes were being killed by the
virtual memory system's OOM killer when in fact there was plentiful
free memory.

Correct a couple minor errors in the sysctl descriptions, and apply
some style fixes.

Reviewed by:	jeff, markj