sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

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