MFV r302662: 6447 handful of nvpair cleanups

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>