Add legacy support to geom raid to create a /dev/arX device for support
of upgrading older machines using ataraid(4) to newer releases.

This optional parameter is controlled via kern.geom.raid.legacy_aliases
and will create a /dev/ar0 device that will point at /dev/raid/r0 for
example.

Tested on Dell SC 1425 DDF-1 format software raid controllers installing from
stable/7 and upgrading to stable/9 without having to adjust /etc/fstab

Reviewed by:	mav
Obtained from:	Yahoo!
MFC after:	2 Weeks
Bring over my initial work from the net80211 TX locking branch.

This patchset implements a new TX lock, covering both the per-VAP (and
thus per-node) TX locking and the serialisation through to the underlying
physical device.

This implements the hard requirement that frames to the underlying physical
device are scheduled to the underlying device in the same order that they
are processed at the VAP layer.  This includes adding extra encapsulation
state (such as sequence numbers and CCMP IV numbers.)  Any order mismatch
here will result in dropped packets at the receiver.

There are multiple transmit contexts from the upper protocol layers as well
as the "raw" interface via the management and BPF transmit paths.
All of these need to be correctly serialised or bad behaviour will result
under load.

The specifics:

* add a new TX IC lock - it will eventually just be used for serialisation
  to the underlying physical device but for now it's used for both the
  VAP encapsulation/serialisation and the physical device dispatch.

  This lock is specifically non-recursive.

* Methodize the parent transmit, vap transmit and ic_raw_xmit function
  pointers; use lock assertions in the parent/vap transmit routines.

* Add a lock assertion in ieee80211_encap() - the TX lock must be held
  here to guarantee sensible behaviour.

* Refactor out the packet sending code from ieee80211_start() - now
  ieee80211_start() is just a loop over the ifnet queue and it dispatches
  each VAP packet send through ieee80211_start_pkt().

  Yes, I will likely rename ieee80211_start_pkt() to something that
  better reflects its status as a VAP packet transmit path.  More on
  that later.

* Add locking around the management and BAR TX sending - to ensure that
  encapsulation and TX are done hand-in-hand.

* Add locking in the mesh code - again, to ensure that encapsulation
  and mesh transmit are done hand-in-hand.

* Add locking around the power save queue and ageq handling, when
  dispatching to the parent interface.

* Add locking around the WDS handoff.

* Add a note in the mesh dispatch code that the TX path needs to be
  re-thought-out - right now it's doing a direct parent device transmit
  rather than going via the vap layer.  It may "work", but it's likely
  incorrect (as it bypasses any possible per-node power save and
  aggregation handling.)

Why not a per-VAP or per-node lock?

Because in order to ensure per-VAP ordering, we'd have to hold the
VAP lock across parent->if_transmit().  There are a few problems
with this:

* There's some state being setup during each driver transmit - specifically,
  the encryption encap / CCMP IV setup.  That should eventually be dragged
  back into the encapsulation phase but for now it lives in the driver TX path.
  This should be locked.

* Two drivers (ath, iwn) re-use the node->ni_txseqs array in order to
  allocate sequence numbers when doing transmit aggregation.  This should
  also be locked.

* Drivers may have multiple frames queued already - so when one calls
  if_transmit(), it may end up dispatching multiple frames for different
  VAPs/nodes, each needing a different lock when handling that particular
  end destination.

So to be "correct" locking-wise, we'd end up needing to grab a VAP or
node lock inside the driver TX path when setting up crypto / AMPDU sequence
numbers, and we may already _have_ a TX lock held - mostly for the same
destination vap/node, but sometimes it'll be for others.  That could lead
to LORs and thus deadlocks.

So for now, I'm sticking with an IC TX lock.  It has the advantage of
papering over the above and it also has the added advantage that I can
assert that it's being held when doing a parent device transmit.
I'll look at splitting the locks out a bit more later on.

General outstanding net80211 TX path issues / TODO:

* Look into separating out the VAP serialisation and the IC handoff.
  It's going to be tricky as parent->if_transmit() doesn't give me the
  opportunity to split queuing from driver dispatch.  See above.

* Work with monthadar to fix up the mesh transmit path so it doesn't go via
  the parent interface when retransmitting frames.

* Push the encryption handling back into the driver, if it's at all
  architectually sane to do so.  I know it's possible - it's what mac80211
  in Linux does.

* Make ieee80211_raw_xmit() queue a frame into VAP or parent queue rather
  than doing a short-cut direct into the driver.  There are QoS issues
  here - you do want your management frames to be encapsulated and pushed
  onto the stack sooner than the (large, bursty) amount of data frames
  that are queued.  But there has to be a saner way to do this.

* Fragments are still broken - drivers need to be upgraded to an if_transmit()
  implementation and then fragmentation handling needs to be properly fixed.

Tested:

* STA - AR5416, AR9280, Intel 5300 abgn wifi
* Hostap - AR5416, AR9160, AR9280
* Mesh - some testing by monthadar@, more to come.