- Take advantage of bus_dmamap_load_mbuf_sg(9).

- Take advantage of bus_dmamap_load_mbuf_sg(9).
- Take advantage of m_collapse(9).
- Sync with other NIC drivers and prepend a TX mbuf if the first attempt
  to load it fails with an error other than EFBIG and stop trying instead
  of freeing it and keeping on trying to enqueue more mbufs. Also ensure
  the driver queue isn't empty before trying to enqueue mbufs in order to
  reduce locking operations.
- In xl_ifmedia_upd() add a missing XL_UNLOCK(). [1]
- Const'ify the xl_devs array.
- Remove an outdated comment.

PR:		113406 [1]
MFC after:	1 month