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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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: let nvlist_recv() pass flags to recv()

Differential Revision: https://reviews.freebsd.org/D32721

lib: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

MFp4 @1189139:

Get rid of the msg_peek() function, which has a problem. If there was less
data in the socket buffer than requested by the caller, the function would busy
loop, as select(2) will always return immediately.

We can just receive nvlhdr now, because some time ago we splitted receive of
data from the receive of descriptors.

MFC after: 1 week

Bring in libnv library for managing name/value pairs. The following types
are currently supported:

- NV_TYPE_NULL - only name, no data;
- NV_TYPE_BOOL - boolean (true or false);
- NV_TYPE_NUMBER - 64bit unsigned integer;
- NV_TYPE_STRING - C string;
- NV_TYPE_NVLIST - nested nvlist;
- NV_TYPE_DESCRIPTOR - file descriptor;
- NV_TYPE_BINARY - binary data.

For detailed documentation and examples see nv(3) manual page.

Sponsored by: The FreeBSD Foundation