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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

libnv: bump library version

Now that we version symbols we should bump the library major version.
While here use version FBSD_1.7 to match the current HEAD FreeBSD
namespace and remove extraneous 'All rights reserved' and incorrect
copyright statement.

Reviewed by: kevans
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D35875

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