em(4): Sync with Intel's em-6.9.6(FreeBSD)

em(4): Sync with Intel's em-6.9.6(FreeBSD)

In addition to adding support for some chips (e.g. 82574L), this also gives
me the chance to rearrange and clean up if_em.[ch] :)

Noticable changes to the FreeBSD driver:
- The hardware abstraction layer is put into a seperate module (ig_hal)
- IP csum offloading is supported when hardware TX csum offloading is enabled
- mbufs on RX/TX ring are freed in em_stop(), i.e. during "ifconfig emX down"
- TX pattern is adjusted that we test number of avaiable TX descs before
  dequeuing mbuf from ifq.  We also reserve double spare TX descs for 82544
  cards on PCI-X bus, so we will not need to unload the loaded mbuf in the
  mid way due to short of TX descs; this at least makes the logic a little
  bit simpler.