hostapd: Settings for DAE/CoA server

hostapd supports "Dynamic Authorization Extensions", making it possible
to forcibly disconnect a user by sending it a RADIUS "Disconnect-Request"
packet.

I've added three new variables to enable setting of the
"radius_das_client" and "radius_das_port" variables in the hostapd
configuration, which enable these extensions.

* dae_client - IP of the client that can send disconnect requests
* dae_secret - shared secret for DAE packets

These are combined into the "radius_das_client" option in hostapd.conf
To enable the server, both dae_client and dae_secret must be set.

* dae_port - optional, default value is 3799 as specified in RFC 5176

Signed-off-by: Martijn van de Streek <martijn@vandestreek.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37734 3c298f89-4303-0410-b956-a3cf2f4a3e73