perfdhcp with late-exit-delay + documentation fix

perfdhcp with late-exit-delay + documentation fix

    * perfdhcp -m <delay> waits for <delay> ms after an exit condition
has been met to receive all packets without sending any new packets.
    * documentation: "Use -D0 to abort if even a single request has
been dropped.". -D0 is not allowed, -D1 achieves that.