Generate random ANonce instead of one based on Counter

Generate random ANonce instead of one based on Counter

Definition of ANonce selection in IEEE Std 802.11i-2004 is somewhat
ambiguous. The Authenticator state machine uses a counter that is
incremented by one for each 4-way handshake. However, the security
analysis of 4-way handshake points out that unpredictable nonces help in
preventing precomputation attacks. Instead of the state machine
definition, use an unpredictable nonce value here to provide stronger
protection against potential precomputation attacks.

Signed-hostap: Jouni Malinen <j@w1.fi>