Forwardport r1950: Use WPA2/AES by default on 2.0n.
The previous default, WPA/TKIP does not allow for speeds greater than
54M (per the 802.11n specification). By changing the default, users can
benefit from high speeds out of the box.
This setting might cause older hardware to stop working, but users can
always downgrade the setting later on. Using WPA/WPA2-Mixed mode
is rumored to also cause problems with some hardware, so we default to
full WPA2 mode now.