- correct a bug in pcap_dispatch(): a count of 0 means infinity.
- correct a bug in pcap_dispatch(): a count of 0 means infinity.

- in pcap_dispatch(), issue a prefetch on the buffer before the
  callback, this may save a little bit of time if the client
  is very fast.

- in pcap_inject(), use a fast copy routine, which also helps
  saving a few nanoseconds with fast clients.

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