netlink: Add sysctl descriptions for net.netlink tree

Reviewed by: markj, melifaro
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43368

netlink: use protocol specific receive buffer

Implement Netlink socket receive buffer as a simple TAILQ of nl_buf's,
same part of struct sockbuf that is used for send buffer already.
This shaves a lot of code and a lot of extra processing. The pcb rids
of the I/O queues as the socket buffer is exactly the queue. The
message writer is simplified a lot, as we now always deal with linear
buf. Notion of different buffer types goes away as way as different
kinds of writers. The only things remaining are: a socket writer and
a group writer.
The impact on the network stack is that we no longer use mbufs, so
a workaround from d18715475071 disappears.

Note on message throttling. Now the taskqueue throttling mechanism
needs to look at both socket buffers protected by their respective
locks and on flags in the pcb that are protected by the pcb lock.
There is definitely some room for optimization, but this changes tries
to preserve as much as possible.

Note on new nl_soreceive(). It emulates soreceive_generic(). It
must undergo further optimization, see large comment put in there.

Note on tests/sys/netlink/test_netlink_message_writer.py. This test
boiled down almost to nothing with mbufs removed. However, I left
it with minimal functionality (it basically checks that allocating N
bytes we get N bytes) as it is one of not so many examples of ktest
framework that allows to test KPIs with python.

Note on Linux support. It got much simplier: Netlink message writer
loses notion of Linux support lifetime, it is same regardless of
process ABI. On socket write from Linux process we perform
conversion immediately in nl_receive_message() and on an output
conversion to Linux happens in in nl_send_one(). XXX: both
conversions use M_NOWAIT allocation, which used to be the case
before this change, too.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D42524

netlink: move NETLINK define to opt_global.h

Move the NETLINK define into opt_global.h so we can rely on it being
set correctly, without having to remember to include opt_netlink.h.
This ensures that the NETLINK define is correctly set. If not we
may end up with unloadable modules, due to missing symbols (such as
nlmsg_get_group_writer).

PR: 274306
Reviewed by: imp, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42179

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

route: show originator PID in netlink monitor

Replacing rtsock with netlink also means providing similar tracing facilities,
rtsock provides `route -n monitor` interface, where each message can be traced
to the originating PID.
This diff closes the feature gap between rtsock and netlink in that regard.

Netlink works slightly differently from rtsock, as it is a generic message
"broker". It calls some kernel KPIs and returns the result to the caller.
Other Netlink consumers gets notified on the changed kernel state using the
relevant subsystem callbacks. Typically, it is close to impossible to pass
some data through these KPIs to enhance the notification.

This diff approaches the problem by using osd(9) to assign the relevant
socket pointer (`'nlp`) to the per-socket taskqueue execution thread.
This change allows to recover the pointer in the aforementioned notification
callbacks and extract some additional data.
Using `osd(9)` (and adding additional metadata) to the notification receiver
comes with some additional cost attached, so this interface needs to be
enabled explicitly by using a newly-created `NETLINK_MSG_INFO` `SOL_NETLINK`
socket option.

The actual medatadata (which includes the originator PID) is provided via
control messages. To enable extensibility, the control message data is
encoded in the standard netlink(TLV-based) fashion. The list of the
currently-provided properties can be found in `nlmsginfo_attrs`.
snl(3) is extended to enable decoding of netlink messages with metadata
(`snl_read_message_dbg()` stores the parsed structure in the provided buffer).

Differential Revision: https://reviews.freebsd.org/D39391

netlink: fix build without NETLINK in the kernel.

PR: 271066

netlink: add netlink interfaces to if_clone

This change adds netlink create/modify/dump interfaces to the `if_clone.c`.
The previous attempt with storing the logic inside `netlink/route/iface_drivers.c`
did not quite work, as, for example, dumping interface-specific state
(like vlan id or vlan parent) required some peeking into the private interfaces.

The new interfaces are added in a compatible way - callers don't have to do anything
unless they are extended with Netlink.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D39032
MFC after: 1 month

netlink: fix 19e43c163c64 by adding miseed netlinkg_glue.c