Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

The current code used the fields in ath_set11nratescenario() . Use them
correctly:

* pass in whether to allow the hardware to override the duration field
in the main data frame (durupdate_en) - PS_POLL frames in particular
don't have the duration bit overriden;
* there's no rts/cts duration here; that's done elsehwere

Include some preliminary TX HT rate scenario setup code.

The AR5416 and later TX descriptors have new fields for supporting
11n bits (eg 20/40mhz mode, short/long GI) and enabling/disabling
RTS/CTS protection per rate.

These functions will be responsible for initialising the TX descriptors
for the AR5416 and later chips for both HT and legacy frames.

Beacon frames will remain using the non-11n TX descriptor setup for now;
Linux ath9k does much the same.

Note that these functions aren't yet used anywhere; a few more framework
changes are needed before all of the right rate information is available
for TX.