Add a new "peer measurement" API, that can be used to measure
certain things related to a peer. Right now, only implement
FTM (flight time measurement) over it, but the idea is that
it'll be extensible to also support measuring the necessary
things to calculate e.g. angle-of-arrival for WiGig.

The API is structured to have a generic list of peers and
channels to measure with/on, and then for each of those a
set of measurements (again, only FTM right now) to perform.

Results are sent to the requesting socket, including a final
complete message.

Closing the controlling netlink socket will abort a running
measurement.

v3:
 - add a bit to report "final" for partial results
 - remove list keeping etc. and just unicast out the results
   to the requester (big code reduction ...)
 - also send complete message unicast, and as a result
   remove the multicast group
 - separate out struct cfg80211_pmsr_ftm_request_peer
   from struct cfg80211_pmsr_request_peer
 - document timeout == 0 if no timeout
 - disallow setting timeout nl80211 attribute to 0,
   must not include attribute for no timeout
 - make MAC address randomization optional
 - change num bursts exponent default to 0 (1 burst, rather
   rather than the old default of 15==don't care)

v4:
 - clarify NL80211_ATTR_TIMEOUT documentation

v5:
 - remove unnecessary nl80211 multicast/family changes
 - remove partial results bit/flag, final is sufficient
 - add max_bursts_exponent, max_ftms_per_burst to capability
 - rename "frames per burst" -> "FTMs per burst"

v6:
 - rename cfg80211_pmsr_free_wdev() to cfg80211_pmsr_wdev_down()
   and call it in leave, so the device can't go down with any
   pending measurements

v7:
 - wording fixes (Lior)
 - fix ftm.max_bursts_exponent to allow having the limit of 0 (Lior)

v8:
 - copyright statements
 - minor coding style fixes
 - fix error path leak
Signed-off-by: Johannes Berg <johannes.berg@intel.com>