sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Remove 12.x ABI compat for kernel dump ioctls

This code was marked gone_in(14), so it can now be removed.

The only consumer of this interface is dumpon(8). We do not maintain
strict backwards compatibility for this utility because a) it
can't/shouldn't be used from a jail or chroot and b) it is highly
specific interface unique to FreeBSD. The host's (presumably more
up-to-date) copy of dumpon(8) should be used to configure kernel dump
devices.

Reviewed by: markj, emaste
MFC after: never
Differential Revision: https://reviews.freebsd.org/D34914

Split out a more generic debugnet(4) from netdump(4)

Debugnet is a simplistic and specialized panic- or debug-time reliable
datagram transport. It can drive a single connection at a time and is
currently unidirectional (debug/panic machine transmit to remote server
only).

It is mostly a verbatim code lift from netdump(4). Netdump(4) remains
the only consumer (until the rest of this patch series lands).

The INET-specific logic has been extracted somewhat more thoroughly than
previously in netdump(4), into debugnet_inet.c. UDP-layer logic and up, as
much as possible as is protocol-independent, remains in debugnet.c. The
separation is not perfect and future improvement is welcome. Supporting
INET6 is a long-term goal.

Much of the diff is "gratuitous" renaming from 'netdump_' or 'nd_' to
'debugnet_' or 'dn_' -- sorry. I thought keeping the netdump name on the
generic module would be more confusing than the refactoring.

The only functional change here is the mbuf allocation / tracking. Instead
of initiating solely on netdump-configured interface(s) at dumpon(8)
configuration time, we watch for any debugnet-enabled NIC for link
activation and query it for mbuf parameters at that time. If they exceed
the existing high-water mark allocation, we re-allocate and track the new
high-water mark. Otherwise, we leave the pre-panic mbuf allocation alone.
In a future patch in this series, this will allow initiating netdump from
panic ddb(4) without pre-panic configuration.

No other functional change intended.

Reviewed by: markj (earlier version)
Some discussion with: emaste, jhb
Objection from: marius
Differential Revision: https://reviews.freebsd.org/D21421

List-ify kernel dump device configuration

Allow users to specify multiple dump configurations in a prioritized list.
This enables fallback to secondary device(s) if primary dump fails. E.g.,
one might configure a preference for netdump, but fallback to disk dump as a
second choice if netdump is unavailable.

This change does not list-ify netdump configuration, which is tracked
separately from ordinary disk dumps internally; only one netdump
configuration can be made at a time, for now. It also does not implement
IPv6 netdump.

savecore(8) is already capable of scanning and iterating multiple devices
from /etc/fstab or passed on the command line.

This change doesn't update the rc or loader variables 'dumpdev' in any way;
it can still be set to configure a single dump device, and rc.d/savecore
still uses it as a single device. Only dumpon(8) is updated to be able to
configure the more complicated configurations for now.

As part of revving the ABI, unify netdump and disk dump configuration ioctl
/ structure, and leave room for ipv6 netdump as a future possibility.
Backwards-compatibility ioctls are added to smooth ABI transition,
especially for developers who may not keep kernel and userspace perfectly
synced.

Reviewed by: markj, scottl (earlier version)
Relnotes: maybe
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D19996

Import the netdump client code.

This is a component of a system which lets the kernel dump core to
a remote host after a panic, rather than to a local storage device.
The server component is available in the ports tree. netdump is
particularly useful on diskless systems.

The netdump(4) man page contains some details describing the protocol.
Support for configuring netdump will be added to dumpon(8) in a future
commit. To use netdump, the kernel must have been compiled with the
NETDUMP option.

The initial revision of netdump was written by Darrell Anderson and
was integrated into Sandvine's OS, from which this version was derived.

Reviewed by: bdrewery, cem (earlier versions), julian, sbruno
MFC after: 1 month
X-MFC note: use a spare field in struct ifnet
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15253