Avoid noisy output from man-db-cache-update triggers
    
    When the man-db-cache-update trigger added in 8e16b24 ("run cache update
    in a transient service using systemd-run - resolves #1318058",
    2017-11-09) is run, it produces potentially confusing output:
    
    Running transaction
      Preparing        :                                           1/1
      Upgrading        : fedpkg-1.31-1.fc26.noarch                 1/2
      Cleanup          : fedpkg-1.30-4.fc26.noarch                 2/2
      Running scriptlet: fedpkg-1.30-4.fc26.noarch                 2/2
    Running as unit: run-re2635381e8fe44a6aad4926eddab6961.service
    Running as unit: run-r8f76223bb81544e8b6f53b8c88b3e72f.service
      Verifying        : fedpkg-1.31-1.fc26.noarch                 1/2
      Verifying        : fedpkg-1.30-4.fc26.noarch                 2/2
    
    The 'Running as unit: run-*.service' output is of little use to users.
    The start of the service is logged to syslog/journal already.  Send
    stdout and stderr of the trigger scriptlet to /dev/null.