lib: Automated cleanup of cdefs and other formatting

Apply the following automated changes to try to eliminate
no-longer-needed sys/cdefs.h includes as well as now-empty
blank lines in a row.

Remove /^#if.*\n#endif.*\n#include\s+<sys/cdefs.h>.*\n/
Remove /\n+#include\s+<sys/cdefs.h>.*\n+#if.*\n#endif.*\n+/
Remove /\n+#if.*\n#endif.*\n+/
Remove /^#if.*\n#endif.*\n/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/types.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/param.h>/
Remove /\n+#include\s+<sys/cdefs.h>\n#include\s+<sys/capsicum.h>/

Sponsored by: Netflix

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Use a platform-independent constant for PKG_MAX_SIZE.

This constant determines the number of rights libnv will attempt to
transmit in a given control message. In practice, the upper limit
defined by the kernel is machine-dependent and is smaller on 64-bit
kernels than on 32-bit kernels. To ensure that a 32-bit libnv works
as expected when run on a 64-bit kernel, use a limit that will work
on both 32-bit and 64-bit kernels.

PR: 238511
Discussed with: oshogbo
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20942

libnv: add support for nvlist_send()/nvlist_recv() on Linux

This may be useful for cross build in the feature.

Submitted by: Mindaugas Rasiukevicius <rmind@noxt.eu>
MFC after: 2 weeks

Allow bootstrapping libnv on macOS and Linux

MacOS/Linux do not define struct cmsgcred but we need to bootstrap libnv
when building on non-FreeBSD systems. Since they are not used during
bootstrap we can just omit these two functions there.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D18472

Ensure that libnv can be used when kern.trap_enotcap=1.

libnv used fcntl(fd, F_GETFL) to test whether fd is a valid file
descriptor. Aside from being racy, this check requires CAP_FCNTL
rights on fd. Instead, use fcntl(fd, F_GETFD), which does not require
any capability rights.

Also remove some redundant fd_is_valid() checks to avoid extra system
calls; in many cases we were performing this check immediately before
dup()ing the descriptor.

Reviewed by: cem, oshogbo (previous version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17963

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.

Remove redundant initialization. Don't use variable - just return the value.
Make scan-build happy by casting to 'void *' instead of 'void **'.

Submitted by: pjd@
MFC after: 1 month
Found by: scan-build and cppcheck
Sponsored by: Wheel Systems

Remove the limit on descriptors that can be send in one nvlist.

Submitted by: Mariusz Zaborski

Assert input arguments to buf_send() and buf_recv().

Submitted by: Mariusz Zaborski <oshogbo@FreeBSD.org>

Fix sending empty nvlist.

Submitted by: Mariusz Zaborski <oshogbo@FreeBSD.org>

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