RTL8812AU / RTL8821AU driver - call for testing

[Andriy Voskoboinyk]

Some updates:
- 11n (A-MPDU & A-MSDU) now is supported (hw.usb.urtwm.enable_11n option  
was removed);
- Atheros Fast-Frames support was added in
https://github.com/s3erios/urtwm/commit/e835e556a0533d0adf81008dd4330241a7a5fbab  
.

> Hi all!
>
> The driver is in https://github.com/s3erios/urtwm ; currently it  
> includes most
> features from urtwn(4) + TCP/UDP/IP checksum validation. 11a is  
> supported too
> (but without DFS at this moment).
>
> Seems to be stable enough with DLINK DWA-171 (RTL8821AU)
> (11a/b/g support was tested only; you can try to enable 11n Rx supportby  
> adding
> "hw.usb.urtwm.enable_11n=1" into /boot/loader.conf).