[net80211] Initial A-MSDU support for testing / evaluation

A-MSDU is another 11n aggregation mechanism where multiple ethernet
frames get LLC encapsulated (so they have a length field), padded,
and put in a single MPDU (802.11 MAC frame.)  This means it gets sent
out as a single frame, with a single seqno, it's acked as one frame, etc.

It turns out that, hah, atheros fast frames is almost but not quite
like this, so I'm reusing all of the current superg/fast-frames stuff
in order to actually transmit A-MSDU.  Yes, this means that A-MSDU
frames are also only aggregated two at a time, so it's not necessarily
a huge win, but it's better than nothing.

This doesn't do anything by default - the driver needs to say it does
A-MSDU as well as set the AMSDU software TX capability so this code path
gets exercised.

For now, the only driver that enables this is urtwn.  I'll enable it
for rsu at some point soon.
Tested:

* Add an amsdu encap path to aggregate two frames, same as the
  fast-frames path.

* Always do the superg init/teardown and node init/teardown stuff,
  regardless of whether the nodes are doing fast-frames (the ATH
  capability stuff.)  That way we can reuse it for amsdu.

* Don't do AMSDU for multicast/broadcast and EAPOL frames.

* If we're doing A-MPDU, then don't bother doing FF/A-MSDU.
  We can likely do both together, but I don't want to change
  behaviour.

* Teach the fast frames approx txtime logic to support the 11n
  rates.  But, since we don't currently have a full "current rate"
  support, assume it's HT20, long-gi, etc.  That way we overshoot
  on the TX time estimation, so we're always inside the requirements.
  (And we only aggregate two frames for now, so we're not really
  going to exceed that.)

* Drop the maximum FF age default down to 2ms, otherwise we end up
  with some very annoyingly large latencies.

TODO:

* We only aggregate two ethernet frames, so I'm not checking the max
  A-MSDU size.  But when it comes time to support >2 frames, we should
  obey that.

Tested:

* urtwn(4)

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