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

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

iflib(9): Remove support for cloning pseudo interfaces

This code was used by the first incarnation of wg(4) and is dead ever
since f187d6dfbf633665ba6740fe22742aec60ce02a2 has removed the latter
again. Moreover, this code matched iflib(4) like a square peg fits in
a round hole, was incomplete and despite some hacks still tailored to
VPC and wg(4) but not generic. In effect, this reverts the following:
09f6ff4f1a47c3009dc16fdc609a44f2341bc7ac (w/ its "ancillary changes")
9aeca21324f481f57f2ecb7009f461f4f51b62b3
1f93e931d9f0c688f43f98ef777e04636a325526
0f9544d03e89d180f94a7a84b110ec7d2b6c625a
0dd691b41276ce13d25ffb1443af27f85038aa3f

Reviewed by: erj, kbowling
Differential Revision: <https://reviews.freebsd.org/D41196>

Fix some modules to export more used symbols

and remove non-present symbols that are now reported by kmod_syms.awk.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32878

Fix `make` in sys/modules

Sponsored by: The FreeBSD Foundation

Make iflib a loadable module: add seemingly missed header.

Reported by: CI (i.e. it is not reproducable in my local builds)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks

Make iflib a loadable module.

iflib is already a module, but it is unconditionally compiled into the
kernel. There are drivers which do not need iflib(4), and there are
situations where somebody might not want iflib in kernel because of
using the corresponding driver as module.

Reviewed by: marius
Discussed with: erj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D19041