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

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

sys/modules: normalize .CURDIR-relative paths to SRCTOP

This simplifies make output/logic

Tested with: `cd sys/modules; make ALL_MODULES=` on amd64
MFC after: 1 month
Sponsored by: Dell EMC Isilon

17 years and change after I wrote warp_saver, here's a simple plasma effect
(currently only three circular patterns) which requires quite a bit of
fixed-point arithmetic, including sqrt() and cos(). Happy New Year!