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

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

namespace nv names, version libnv and libnvpair library symbols

libnv and libnvpair have aliased symbols, and as a result a single process which
dlopens a shared object that is dynamically linked to libnv and another to
libnvpair will wind up with a single set of resolved symbols for those in
conflict. A source file also cannot include both libnv and libnvpair headers
because of aliased identifiers. To resolve the situation, libnv types and
functions are namespaced via nv_namespace.h, and libnv symbols are
versioned. The msgio functions are not namespaced or exported as they are not
part of the external API.

Reviewed by: kevans
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35261

sys: clean up empty lines in .c and .h files

libnv: Remove nvlist argument from cnvlist_{take,free}_* functions.

All information which are need for those operations is already stored in
the cookie.

We decided not to bump libnv version because this API is not used yet in the
base system.

Reviewed by: pjd

libnv: add const to cookies arguments

Pointed out by: pjd@

libnv: change name of cookie from cookiep to cookie.

The name was inconsistent with rest of the library.
No functional change intended.

Pointed out by: pjd@

SPDX: mostly fixes to previous changes.

Introduce the recently approved BSD-1-Clause and replace 0BSD which
never did fit well our use cases.

Introduce cnvlist_name() and cnvlist_type() functions.

Those function can be used when we are iterating over nvlist to reduce
amount of extra variables we need to declare.

MFC after: 1 month

Remove duplicated declaration.

Reported by: adrian@

Add missed header file for cnv.h .

Submitted by: Adam Starak <starak.adam@gmail.com>
Reported by: ache@