(#17029) add osfamily Gentoo

With a single member of operatingsystem type Gentoo. Tests for popular
Gentoo derivatives like Sabayon are not current in the facter tree so
this patch is not adding them to the the initial Gentoo osfamily
definition.
(#17521) Add osfamily fact ArchLinux

Archlinux doesn't have a osfamily fact.

I believe Archlinux needs one because of the following:

* Manjaro Linux based on Archlinux http://blog.manjaro.org/
* Archbang based on Archlinux http://archbang.org/
* More archlinux based distro's:
  https://wiki.archlinux.org/index.php/Arch_Based_Distributions_(Active)

Conflicts:
	lib/facter/osfamily.rb