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

Kill MFILES and find things automatically. It turned out to be only
lightly used. Find the proper .m file when we depend on *_if.[ch] in
the srcs line, with seat-belts for false positive matches. This uses
make's path mechanism. A further refinement would be to calculate this
once, and then pass the resulting _MPATH to modules submakes.

Differential Revision: https://reviews.freebsd.org/D2327

Import virtio base, PCI front-end, and net/block/balloon drivers.
Tested on Qemu/KVM, VirtualBox, and BHyVe.

Currently built as modules-only on i386/amd64. Man pages not yet hooked
up, pending review.

Submitted by: Bryan Venteicher bryanv at daemoninthecloset dot org
Reviewed by: bz
MFC after: 4 weeks or so