wlan - Sync dev/netif/bwn/bwn from FreeBSD

wlan - Sync dev/netif/bwn/bwn from FreeBSD

* Sync dev/netif/bwn/bwn from FreeBSD, fbsd git dd885b9a0a0e, May 11 2016.

* Straight report from FreeBSD using new infrastructure.

* Add bwn back to build

* Using contigmalloc for allocating dr_txhdr_cache, because kmalloc on
  DragonFly wouldn't always give us physically contiguous memory.

* Keeping the old code for (MSI) interrupt allocation, which puts things
  into the softc struct, and not into the bwn_mac struct like the new code.

* XXX Not tested yet, also some local changes might be missing still.
* XXX Didn't sync dev/netif/bwn/siba_bwn from FreeBSD yet, but bwn might
      already work for the previously supported chipsets like this.