spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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: add netlink KPI to the kernel by default

This change does the following:

Base Netlink KPIs (ability to register the family, parse and/or
write a Netlink message) are always present in the kernel. Specifically,
* Implementation of genetlink family/group registration/removal,
some base accessors (netlink_generic_kpi.c, 260 LoC) are compiled in
unconditionally.
* Basic TLV parser functions (netlink_message_parser.c, 507 LoC) are
compiled in unconditionally.
* Glue functions (netlink<>rtsock), malloc/core sysctl definitions
(netlink_glue.c, 259 LoC) are compiled in unconditionally.
* The rest of the KPI _functions_ are defined in the netlink_glue.c,
but their implementation calls a pointer to either the stub function
or the actual function, depending on whether the module is loaded or not.

This approach allows to have only 1k LoC out of ~3.7k LoC (current
sys/netlink implementation) in the kernel, which will not grow further.
It also allows for the generic netlink kernel customers to load
successfully without requiring Netlink module and operate correctly
once Netlink module is loaded.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39269

netlink: allow netlink sockets in non-vnet jails.

This change allow to open Netlink sockets in the non-vnet jails, even for
unpriviledged processes.
The security model largely follows the existing one. To be more specific:
* by default, every `NETLINK_ROUTE` command is **NOT** allowed in non-VNET
jail UNLESS `RTNL_F_ALLOW_NONVNET_JAIL` flag is specified in the command
handler.
* All notifications are **disabled** for non-vnet jails (requests to
subscribe for the notifications are ignored). This will change to be more
fine-grained model once the first netlink provider requiring this gets
committed.
* Listing interfaces (RTM_GETLINK) is **allowed** w/o limits (**including**
interfaces w/o any addresses attached to the jail). The value of this is
questionable, but it follows the existing approach.
* Listing ARP/NDP neighbours is **forbidden**. This is a **change** from the
current approach - currently we list static ARP/ND entries belonging to the
addresses attached to the jail.
* Listing interface addresses is **allowed**, but the addresses are filtered
to match only ones attached to the jail.
* Listing routes is **allowed**, but the routes are filtered to provide only
host routes matching the addresses attached to the jail.
* By default, every `NETLINK_GENERIC` command is **allowed** in non-VNET jail
(as sub-families may be unrelated to network at all).
It is the goal of the family author to implement the restriction if
necessary.

Differential Revision: https://reviews.freebsd.org/D39206
MFC after: 1 month

netlink: add public ucred accessor for nlp.

MFC after: 2 weeks

netlink: make test-includes happy by hiding most of the header
contents under _KERNEL.

netlink: add netlink support

Netlinks is a communication protocol currently used in Linux kernel to modify,
read and subscribe for nearly all networking state. Interfaces, addresses, routes,
firewall, fibs, vnets, etc are controlled via netlink.
It is async, TLV-based protocol, providing 1-1 and 1-many communications.

The current implementation supports the subset of NETLINK_ROUTE
family. To be more specific, the following is supported:
* Dumps:
- routes
- nexthops / nexthop groups
- interfaces
- interface addresses
- neighbors (arp/ndp)
* Notifications:
- interface arrival/departure
- interface address arrival/departure
- route addition/deletion
* Modifications:
- adding/deleting routes
- adding/deleting nexthops/nexthops groups
- adding/deleting neghbors
- adding/deleting interfaces (basic support only)
* Rtsock interaction
- route events are bridged both ways

The implementation also supports the NETLINK_GENERIC family framework.

Implementation notes:
Netlink is implemented via loadable/unloadable kernel module,
not touching many kernel parts.
Each netlink socket uses dedicated taskqueue to support async operations
that can sleep, such as interface creation. All message processing is
performed within these taskqueues.

Compatibility:
Most of the Netlink data models specified above maps to FreeBSD concepts
nicely. Unmodified ip(8) binary correctly works with
interfaces, addresses, routes, nexthops and nexthop groups. Some
software such as net/bird require header-only modifications to compile
and work with FreeBSD netlink.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D36002
MFC after: 2 months