Add a generic Proportional Integral Derivative (PID) controller algorithm and
use it to regulate page daemon output.

This provides much smoother and more responsive page daemon output, anticipating
demand and avoiding pageout stalls by increasing the number of pages to match
the workload. This is a reimplementation of work done by myself and mlaier at
Isilon.

Reviewed by: bsdimp
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14402