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

Add VirtIO console driver

Support for the multiport feature is mostly implemented, but currently
disabled due to some potential races in the hot plug code paths.

Requested by: marcel
MFC after: 1 month
Relnotes: yes