MFC r304908-r304909

r304908:
Add cnv API.

cnv API is a set of functions for managing name/value pairs by cookie.
The cookie can be obtained by nvlist_next(), nvlist_get_parent() or
nvlist_get_pararr() function. This patch also includes unit tests.

Submitted by: Adam Starak <starak.adam@gmail.com>

r304909:
Fix style issue in the cnv API.
Remove unused arguments in a macro.
Remove unused typedef.