sys: Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

routing: add public rt_is_exportable() version to check if
the route can be exported to userland when jailed.

Differential Revision: https://reviews.freebsd.org/D39204
MFC after: 2 weeks

ifnet/API: Move struct ifnet definition to a <net/if_private.h>

Hide the ifnet structure definition, no user serviceable parts inside,
it's a netstack implementation detail. Include it temporarily in
<net/if_var.h> until all drivers are updated to use the accessors
exclusively.

Reviewed by: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38046

routing: Use NET_EPOCH_[CALL|WAIT] macros

Reviewed by: melifaro, kp
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D37729

routing: extend nhop(9) kpi

* add nhop_get_unlinked() used to prepare referenced but not
linked nexthop, that can later be used as a clone source.
* add nhop_check_gateway() to check for allowed address family
combinations between the rib family and neighbor family (useful
for 4o6 or direct routes)
* add nhop_set_upper_family() to allow copying IPv6 nexthops to
IPv4 rib.
* add rt_get_rnd() wrapper, returning both nexthop/group and its
weight attached to the rtentry.
* Add CHT_SLIST_FOREACH_SAFE(), allowing to delete items during
iteration.

MFC after: 2 weeks

routing: move rtentry and subscription code out of route_ctl.c

route_ctl.c size has grown considerably since initial introduction.
Factor out non-relevant parts:
* all rtentry logic, such as creation/destruction and accessors
goes to net/route/route_rtentry.c
* all rtable subscription logic goes to net/route/route_subscription.c

Differential Revision: https://reviews.freebsd.org/D36074
MFC after: 1 month