net80211 rate control API - 802.11n extensions

by Chenchong Qin for FreeBSD

There is a simplistic rate control API in net80211 of FreeBSD, which lack the support of 802.11n features. 802.11n brought a 10x maximum net data rate compared to its predecessor, but, unfortunately, the hard-won rate up can be easily wasted if rate control hasn't been properly performed. This project will extend the net80211 rate control API of FreeBSD to be 802.11n aware and be able to support multiple rate attempts. With the extended API, wireless throughput can be further imrpoved.