NOACK on outgoing Radiotap packets

Hi, list

I have been working a bit with the ath5k driver which is a mac80211 driver.
It is now possible for mac80211 based drivers to send packets via the
monitoring interface. Such packets must however be marked for NOACK in the
tx flags, because otherwise, the driver will expect an ACK and thus
retransmit all packets. I have made a pull request that provides an element
SetNoAck() to set this.

Below is an example:

RatedSource() ->
UDPIPEncap(10.0.0.1, 1001, 10.0.0.2, 1002) ->
EtherEncap(0x0800, 2:0:0:0:0:1, 2:0:0:0:0:2) ->
WifiEncap(0, 00:00:00:00:00:00) ->
SetTXRate(3) ->
SetNoAck() ->
RadiotapEncap() ->
Queue() ->
ToDevice(mon0);

Setting TX power with SetTxPower does not have any effect on this kind of
radio.

Setting TX rate, can be done with the SetTxRate() element. However, if the
mac80211 radio has rate control, that will override the rate set by
SetTxRate. Below is a small patch for mac80211 that changes the behavior so
that rate control is not applied if the packet already got a rate.