[WIP] substitute pkgng for pkg_info, pkg_add, and such

"pkgng" is the new package manager. see
http://www.freebsd.org/doc/en/books/handbook/pkgng-intro.html

I suppose it should use pkgng instead of pkg_info, pkg_add, and such. pkgng
supports FreeBSD 9.1 over, and moreover, pkg_info and such have been removed
since FreeBSD 10.0-CURRENT.

see also: https://github.com/serverspec/serverspec/pull/348