sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

nlsysevent: remove useless CFLAGS

nlsysevent: Fix the EXPORT_SYMS definition

EXPORT_SYMS=YES has a special meaning, EXPORT_SYMS=yes does not.

Fixes: 8a2af0b469b6 ("nlsysevent: add a genetlink(4) module to report kernel events")

nlsysevent: add a genetlink(4) module to report kernel events

Hooked to devctl_notify, this allows consumers to received events
by subscribing to a system over a generic netlink protocol

Reviewed by: imp, melifaro
Differential Revision: https://reviews.freebsd.org/D37574