Add a hostapd testing option for skipping association pruning

Add a hostapd testing option for skipping association pruning

The new skip_prune_assoc=1 parameter can be used to configure hostapd
not to prune associations from other BSSs operated by the same process
when a station associates with another BSS. This can be helpful in
testing roaming cases where association and authorization state is
maintained in an AP when the stations returns.

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