(PUP-3618) Include pkgng provider for FreeBSD

To improve the default state of Puppet on FreeBSD systems, !his work
moves the pkgng provider into core.  Previously, users of the provider
would need to install from the forge module zleslie/pkgng.

This was a community effort:

    Alex Hornung <alex@alexhornung.com>
    Athanasios Douitsis <aduitsis@cpan.org>
    Jiří Kubíček <jiri.kubicek@kraxnet.cz>
    Reinier Schoof <reinier@skoef.nl>
    Romain Tartière <romain@blogreen.org>
    Russell Jackson <raj@csub.edu>
    Seth Lyons <slyons@fxcm.com>

Thanks to all who helped grow this provider!