Add RA, a new 11n Tx rate adaptation module for net80211.

Written by Christian Ehrhardt and myself, based on ieee80211_mira.c
but with significant changes.

The main difference is that RA does not attempt to precisely measure
actual throughput but simply deducts a loss percentage from the
theoretical throughput which can be achieved by a given MCS.

Unlike MiRa, RA does not use timeouts to trigger probing.
Probing is triggered only by changes in measured throughput.

Unlike MiRA, RA doesn't care whether a frame was part of an A-MPDU.
RA simply collects statistics for individual subframes. This makes reporting
very easy for drivers and seems to work well enough in practice.

Another difference is that drivers can report multi-rate retries properly
via ieee80211_ra_add_stats_ht(mcs, total, fail) which can be called
several times before ieee80211_ra_choose() selects a new Tx rate.

There is no reason any issues could not be fixed in ieee8011_mira.c but
I felt it was a good moment to burn the house down and start over.
And since this code diverges from how MiRA is described in the research
paper applying the "MiRA" label becomes inappropriate.

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