- Includes unit test parsing and confirming an S1G beacon
 - Includes support for the new Frame Control format for type=3,
   subtype=1 (S1G beacon)
 - Includes changes to support addressing used in S1G beacon.

All dot11 unit tests pass with this change.

* Add support for S1G beacon to the dot11 layer.
 - Includes unit test parsing and confirming an S1G beacon
 - Includes support for the new Frame Control format for type=3,
   subtype=1 (S1G beacon)
 - Includes changes to support addressing used in S1G beacon.

All dot11 unit tests pass with this change.

* Fix review comments from @p-l-:
 - Use set for 'type in' checks for extension frame type 3, subtype 1
   (S1G beacon).

* Update to FCfield to split into three parts for type 3, subtype 1 (S1G
beacon frame control field). This allows use of BitField for the 'bw',
and FlagsField for the remaining bits.