fix the package_update_command to actually do a package upgrade

With pkgin, the 'upgrade' command doesn't actually do what you expect it to. There is a concept of kept and unkept packages. In truth, pkgin is divergent in how it thinks about upgrades with most all other package managers. The philosophy aside, this fix enables the cfengine package_policy  =>  "add update" to work as expected and upgrade the installed package to latest version.