A couple of changes related to ixgbe operation in netmap mode:

- add a sysctl, dev.netmap.ix_crcstrip, to control whether ixgbe should
  strip the CRC on received frames. Defaults to 0, which keeps the CRC.
  and improves performance when receiving min-sized (64-byte) frames.
  This matters because  min-sized frames is one of the standard
  benchmarks for switches and routers, some chipsets seem to issue
  read-modify-write cycles for PCIe transactions that are not a
  full cache line, and a min-sized frame triggers the bug, resulting
  in reduced throughput -- 9.7 instead of 14.88 Mpps -- and heavy
  bus load.

- for the time being, always look for incoming packets on a select/poll
  even if there has not been an interrupt in the meantime. This is
  only a temporary workaround for a probable race condition in keeping
  track of rx interrupts.
  Add a couple of diagnostic vars to help studying the problem.

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