rework NM-dispatcher/dhclient detection
    
    The packaged NM dispatcher and dhclient scripts provide the same
    functionality. Only one should be active depending on the system
    configuration.
    
    Instead of guessing in the NM dispatcher script whether the dhclient
    script is executed, check in the dhclient script whether it is called
    from a NM dispatcher script (the 11-dhclient script included in the
    dhcp-client package).
    
    This is simpler and it removes dependency on the 11-dhclient script.