bwn: Porting from FreeBSD

bwn: Porting from FreeBSD

* Adding bwn and siba_bwn to sys/conf/files and to the X86_64_GENERIC kernel
  configuration.

* Replacing BWN_LOCK, BWN_UNLOCK, BWN_ASSERT_SERIALIZED with
  wlan_global_serializer.

* Increasing some alignments from 4K to 8K and some from 1 byte to 4 bytes.

* Changing slot counts for RX and TX from 64 to 256, which is the same size
  as used by b43 in Linux.

* Allow overriding the BWN_TXRING_SLOTS and BWN_RXRING_SLOTS values in the
  kernel configuration.

* Some style fixes. Removing unneeded NULL checks.

* Replace a DELAY(50000) with tsleep.

* Can't use kmalloc to allocate memory for dma like in FreeBSD and Linux.

* A few typo and grammar fixes in the bwn.4 manpage.