- Define 802.11 modulation types as 'enum ieee80211_modtype'.

- Define 802.11 modulation types as 'enum ieee80211_modtype'.
- Expose ieee80211_rate2modtype() for pubic use.
- Add definition for DIFS, slot time and contention window.
- Add addition field in TX rate control state structure, so drivers can
  give hints to TX rate control algorithms about their capabilities.
- Add Sample TX rate control support:
  http://www.pdos.lcs.mit.edu/papers/jbicket-ms.pdf
  It is factored out and adapted from the one in ath(4).
- In ieee80211_ratectl.h, expose only IEEE80211_RATECTL_{ONOE,AMRR,SAMPLE}
  for user space program.
- Teach ifconfig(8) to show and set Sample TX rate control algorithm.
- Fix a node leakage on rt2560_tx_mgt() error handling path.
- Support Onoe and Sample TX rate control algorithm in 2560 part of
  ral(4), and use Sample TX rate control algorithm as the default TX
  rate control algorithm. [*]
- Make ral(4) depend on wlan_ratectl_{onoe,sample}.
- Hook Sample TX rate control algorithm into GENERIC and LINT.

# [*]
# If Sample TX rate control algorithm is used, I get almost 100~200%
# UDP_STREAM netperf TX performance boost than the original TX rate
# control algorithm in open/noisy enviroments, and +200~500Kbits/s
# UDP_STREAM netperf TX performance boost under good conditions.