spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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: Allow use in non-sleepable contexts

44c125c4cebc2fd87c6260b90eddae11201f5232 switched the nvlist allocations
to be M_WAITOK, but this precludes the use in non-sleepable contexts.
(E.g. with a nonsleepable lock held).

All callers for these allocation functions already cope with memory
alloation failures, so there's no reason to allow sleeping during
allocations.

Reviewed by: melifaro, oshogbo
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29556

libnv: fix memory leaks

nvpair_create_stringv: free the temporary string; this fix affects
nvlist_add_stringf() and nvlist_add_stringv().

nvpair_remove_nvlist_array (NV_TYPE_NVLIST_ARRAY case): free the chain
of nvpairs (as resetting it prevents nvlist_destroy() from freeing it).
Note: freeing the chain in nvlist_destroy() is not sufficient, because
it would still leak through nvlist_take_nvlist_array(). This affects
all nvlist_*_nvlist_array() use

Submitted by: Mindaugas Rasiukevicius <rmind@netbsd.org>
Reported by: clang/gcc ASAN
MFC after: 2 weeks

libnv: Add nvlist_append_*_array() family of functions.

The nvlist_append_{bool,number,string,nvlist,descriptor}_array() functions
allows to dynamically extend array stored in the nvlist.

Submitted by: Mindaugas Rasiukevicius <rmind@netbsd.org>

Add SPDX tags for nv(9).

MFC after: 2 weeks

libnv: Use mallocarray(9) for the nv_calloc.

Add support for the arrays in nvlist library.

- Add
nvlist_{add,get,take,move,exists,free}_{number,bool,string,nvlist,
descriptor} functions.
- Add support for (un)packing arrays.
- Add the nvl_array_next field to the nvlist structure.
If an array is added by the nvlist_{move,add}_nvlist_array function
this field will contains next element in the array.
- Add the nitems field to the nvpair and nvpair_header structure.
This field contains number of elements in the array.
- Add special flag (NV_FLAG_IN_ARRAY) which is set if nvlist is a part of
an array.
- Add special type (NV_TYPE_NVLIST_ARRAY_NEXT).This type is used only
on packing/unpacking.
- Add new API for traversing arrays (nvlist_get_array_next).
- Add the nvlist_get_pararr function which combines the
nvlist_get_array_next and nvlist_get_parent functions. If nvlist is in
the array it will return next element from array. If nvlist is last
element in array or it isn't in array it will return his
container (parent). This function should simplify traveling over nvlist.
- Add tests for new features.
- Add documentation for new functions.
- Add my copyright.
- Regenerate the sys/cddl/compat/opensolaris/sys/nvpair.h file.

PR: 191083
Reviewed by: allanjude (doc)
Approved by: pjd (mentor)

The nvlist_move_nvpair() function can fail in two cases, if:
- the nvlist error is set, or
- the nvlist case ignore flag is not set and there is attend to
add element with duplicated name.
In both cases the nvlist_move_nvpair() function free nvpair structure.
If library will try to unpack a binary blob which contains duplicated
names it will end up with using memory after free.

To prevent that, the nvlist_move_nvpair() function interface is changed
to report about failure and checks are added to the nvpair_xunpack()
function.

Discovered thanks to the american fuzzy lop.

Approved by: pjd (mentor)

Move the nvlist source and private includes from sys/kern to seperate
directory sys/contrib/libnv.

The goal of this operation is to NOT install header files which shouldn't
be used outside the nvlist library.

Approved by: pjd (mentor)