hostapd: Add mechanism to track unconnected stations

hostapd: Add mechanism to track unconnected stations

hostapd can now be configured to track unconnected stations based on
Probe Request frames seen from them. This can be used, e.g., to detect
dualband capable station before they have associated. Such information
could then be used to provide guidance on which colocated BSS to use in
case of a dualband AP that operates concurrently on multiple bands under
the control of a single hostapd process.

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