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

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

LinuxKPI: Constantly use _LINUXKPI_ prefix in include guards

MFC after: 1 week
Reviewed by: bz, emaste, hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33562

LinuxKPI: add Exponentially Weighted Moving Average implementation

Add DECLARE_EWMA() which expands to a per-name EWMA implementation
as used by multiple wireless drivers.

Sposnored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: hselasky, cperciva, dwmalone
Differential Revision: https://reviews.freebsd.org/D30415