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/

libnv: add regression test for r335344.

Remove free'ing of an uninitialized variable

Just remove it completely from the test as it's initialized but unused apart
from the free(3) call

Differential Revision: https://reviews.freebsd.org/D4769 (part of larger diff)
MFC after: 5 days
Reported by: cppcheck
Reviewed by: oshogbo
Sponsored by: EMC / Isilon Storage Division

Use `nitems(x)` macro instead of using hardcoded numbers for indices into
the nvlists

Convert some of the variables from int to unsigned int to squelch -Wsign-compare
warnings when converting hardcoded values to nitems(..)

Differential Revision: https://reviews.freebsd.org/D4769 (part of larger diff)
MFC after: 5 days
Reviewed by: oshogbo
Sponsored by: EMC / Isilon Storage Division

Convert another `string` variable to `string_arr` missed in r293130

Differential Revision: https://reviews.freebsd.org/D4769 (part of larger diff)
MFC after: 5 days
Reviewed by: oshogbo
Sponsored by: EMC / Isilon Storage Division

Rename nitems and string variables to avoid collisions

Rename the `nitems` variable to `num_items` to avoid collisions with the
macro in sys/param.h for counting elements in an array

Similarly, rename `string` to `string_arr` to avoid future collisions with
potential keywords, as well as make it clear that `string_arr` isn't a char*
value, but instead a char** value.

Differential Revision: https://reviews.freebsd.org/D4769 (part of larger diff)
MFC after: 5 days
Reviewed by: oshogbo
Sponsored by: EMC / Isilon Storage Division

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)