(MAINT) Switch bare 'sysctl' call to use Facter::Util::POSIX.sysctl

Well, the intent is still good :).  Perhaps we should replace `uptime_sysctl_cmd` with `uptime_sysctl_variable` that returns just 'kern.boottime' and `Facter::Core::Execution.execute("#{uptime_sysctl_cmd} 2>/dev/null", :on_fail => nil)` with `POSIX.sysctl(uptime_sysctl_variable)`?