Pull up following revision(s) (requested by msaitoh in ticket #1027):
sys/dev/pci/if_wm.c: revision 1.586
sys/dev/pci/if_wm.c: revision 1.587
sys/dev/pci/if_wm.c: revision 1.588

- Split txdrop evcnt into pcqdrop and descdrop.
- Simplify evcnt name by removing duplicated "tx" or "rx".
- s/tu/underrun/

- Try m_defrag() to reduce the number of DMA segment if bus_dmamap_load_mbuf()
  returned EFBIG. When m_defrag() is called, txqNNdefrag event counter is
  incremented. If the 2nd try of bus_dmamap_load_mbuf() failed, txqNNtoomanyseg
  event counter is incremented.

- Reduce the max number of DMA segments from 256 to 64 (it's the same value
  as other BSD's (EM_MAX_SCATTER) and more than before if_wm.c rev. 1.75's
  value (40)) because we do m_defrag() now.

  Fix comment.

(martin)