Import OpenBSD's rsu(4) WLAN driver.
Support chipsets are the Realtek RTL8188SU, RTL8191SU, and RTL8192SU.

Many thanks to Idwer Vollering for porting/writing the man page and for
testing.

Reviewed by:	adrian, hselasky
Obtained from:	OpenBSD
Tested by:	kevlo, Idwer Vollering <vidwer at gmail.com>