MFC r305211: MFV r302662: 6447 handful of nvpair cleanups

illumos/illumos-gate@759e89be359f2af635e4122d147df56bce948773
https://github.com/illumos/illumos-gate/commit/759e89be359f2af635e4122d147df56bc
e948773

https://www.illumos.org/issues/6447
I got a patch from someone who uses nvpair code outside of illumos. It fixes a
couple of gcc warnings/bugs for him.
1. silence uninitialized use warnings
2. add parentheses around assignment used as truth value
3. fix printf format specifier (ll is for integers only)
4. strstr, strspn, strcspn, and strcmp are declared in string.h, not
strings.h.
5. avoid scanning integer into boolean variable

Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Steve Dougherty <sdougherty@barracuda.com>

MFC r296528: MFV r296527: 6659 nvlist_free(NULL) is a no-op

Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Marcel Telka <marcel@telka.sk>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>

illumos/illumos-gate@aab83bb83be7342f6cfccaed8d5fe0b2f404855d

MFC nv(3) and part of nv(9) to stable/10

This includes the following revisions from head:

r258065,r258594,r259430,r260222,r261407,r261408,r263479,r264021,r266351,
r269603,r271026,r271027,r271028,r271241,r271578,r271579,r271847,r272102,
r272843,r273752,r277920,r277921,r277925,r277926,r277927,r279421,r279422,
r279423,r279424,r279425,r279426,r279427,r279428,r279429,r279430,r279431,
r279432,r279434,r279435,r279436,r279438,r279439,r279440,r279760,r282122,
r282254,r282257,r282304,r282312,r285339,r288340

This change reverts stable/10@r282122 and stable/10@r288340, and re-MFCs the
series again (r282122, r285339, and r288340).

More changes are pending to nv(9)/pci(4) after further review/work. Please see
the Phabricator review for more details (both https://reviews.freebsd.org/D4232
and https://reviews.freebsd.org/D4249 ).

- Tested with:
-- Booting VMware Fusion 8.1.0 running on a Haswell Apple Macbook Pro
-- Booting a Haswell machine with zfs and running some stress workloads with
VirtualBox guests
-- make tinderbox
-- kyua test -k /usr/tests/lib/libnv

Differential Revision: https://reviews.freebsd.org/D4249 (part of a larger diff)
Relnotes: yes
Reviewed by: oshogbo (implicit), sbruno (implicit)
Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
Sponsored by: EMC / Isilon Storage Division