taxonomy: Store Probe Request frames in hostapd_sta_info

taxonomy: Store Probe Request frames in hostapd_sta_info

A weakness in the initial client taxonomy mechanism is from storing both
the Probe and Associate in struct sta_info. struct sta_info is created
after a client associates (or starts authentication frame exchange),
which means that any Probe Request frames sent prior to association are
not retained. The Associate Request frame has to be seen, and then
another Probe Request frame after association, before we have a
signature for the client.

Most clients send lots of Probe Request frames (lots and lots and lots
of Probes, actually), but a few do not. ChromeOS is notably sparing in
sending Probe Request frames, it can take a long time before a signature
for a ChromeOS device is available.

Store the most recent Probe Request frame in struct hostapd_sta_info
tracking list. When a struct sta_info is created, move the Probe Request
frame information from struct hostapd_sta_info to struct sta_info.

Signed-off-by: dgentry@google.com (Denton Gentry)
Signed-off-by: denny@geekhold.com (Denton Gentry)
Signed-off-by: rofrankel@google.com (Richard Frankel)
Signed-off-by: richard@frankel.tv (Richard Frankel)