Passive Client Taxonomy

Passive Client Taxonomy

Implement the signature mechanism described in the paper
"Passive Taxonomy of Wifi Clients using MLME Frame Contents"
published by Denton Gentry and Avery Pennarun.

http://research.google.com/pubs/pub45429.html
https://arxiv.org/abs/1608.01725

This involves:
1. Add a CONFIG_TAXONOMY compile option. Enabling taxonomy incurs
   a memory overhead of up to several kilobytes per associated
   station.
2. If enabled, store the Probe Request and (Re)Associate Request frame in
   struct sta_info.
3. Implement code to extract the ID of each Information Element,
   plus selected fields and bitmasks from certain IEs, into a
   descriptive text string. This is done in a new source file,
   src/ap/taxonomy.c.
4. Implement a "signature qq:rr:ss:tt:uu:vv" command
   in hostapd_cli to retrieve the signature.

Signatures take the form of a text string. For example, a signature
for the Nexus 5X is:
  wifi4|probe:0,1,127,45,191,htcap:01ef,htagg:03,htmcs:0000ffff,vhtcap:338061b2,
  vhtrxmcs:030cfffa,vhttxmcs:030cfffa,extcap:00000a0201000040|assoc:0,1,48,45,
  221(0050f2,2),191,127,htcap:01ef,htagg:03,htmcs:0000ffff,vhtcap:339071b2,
  vhtrxmcs:030cfffa,vhttxmcs:030cfffa,extcap:0000000000000040

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)