Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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>