Send dmidecode errors to /dev/null in the virtual fact
    
    When run as a non-root user, facter would always spew "/dev/mem:
    Permission denied" because it called dmidecode without redirecting
    stderr.
    
    This was filed upstream as FACT-86¹.  It's unclear what is insufficient
    about this method, but failing tests that are not visible aren't as much
    of a concern to me as clearly useless output on every run of the
    command.
    
    ¹ https://tickets.puppetlabs.com/browse/FACT-86