/etc/sysconfig/rhn/systemid stores the identity of a system registered
with the Red Hat Network. Previously, yum-rhn-plugin did not allow for
situations where this file was missing, so would cause yum to crash when
it could not find the file. If the systemid file is missing, the plugin
now warns the user that the SystemID could not be acquired and that the
system might not be registered on the Red Hat Network.