Support '%packages --multilib' in dnfpayload.py (#1192628)
    
    The /etc/dnf/dnf.conf syntax is similar to yum's, but dnfpayload.py
    doesn't write out a temporary configuration file.  It just creates a
    configuration object and changes the values.  So do that for
    multilib_policy.
    
    Signed-off-by: David Cantrell <dcantrell@redhat.com>