MFC r326276:

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified 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.

No functional change intended.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Use nitems() from sys/param.h.

MFC after: 2 weeks.

kldconfig: Remove some code for compatibility with FreeBSD 4.x.

Verified using md5(1).

Fix malloc size (visible on amd64, with "kldconfig -r").

Sync program's usage() with manpage's SYNOPSIS.

Make NULL a (void*)0 whereever possible, and fix the warnings(-Werror)
that this provokes. "Wherever possible" means "In the kernel OR NOT
C++" (implying C).

There are places where (void *) pointers are not valid, such as for
function pointers, but in the special case of (void *)0, agreement
settles on it being OK.

Most of the fixes were NULL where an integer zero was needed; many
of the fixes were NULL where ascii <nul> ('\0') was needed, and a
few were just "other".

Tested on: i386 sparc64

Use __FBSDID() to quiet GCC 3.3 warnings.

style(9): use NULL when assigning/comparing a char *,
consitently compare fileid with 0 using == and !=.

Reviewed by: silence on @audit

Properly cast a size argument to an unsigned type.

Thanks to: dd for noticing the need for a cast.

Silence warnings on the Alpha: don't assume size_t is an int.

Add kldconfig(8), a utility to modify the kernel module search path.

Reviewed by: -arch, -audit