fix manual pages for usb_modeswitch
    
    - remove obsolete patch usb_modeswitch-dir.patch.
    - add a manual page for usb_modeswitch_dispatcher (rh #884203).
    - fix several inconsistencies in the manual page and the
      help output of usb_modeswitch (rh #948451).
    
    Resolves: #948451
    Resolves: #884203