Add PBCC modulation support in netproto/802_11:

Add PBCC modulation support in netproto/802_11:
- Since ieee80211com.ic_caps is already crowded, add ieee80211com.ic_caps_ext
  to extend it.  Currently only IEEE80211_CEXT_PBCC, which should be set by
  the drivers capable of PBCC modulation, is defined for this field.
- Adapt SIOCG80211:IEEE80211_IOC_DRIVER_CAPS ioctl handling to expose the
  content of ieee80211com.ic_caps_ext.  Old semantic of this ioctl is not
  changed, given ieee80211req is zeroed out before the ioctl call.
- Add debug information about peer node's rate set (after negociation)
- Set PBCC bit in Capability ie base on the value of
  (ieee80211com.ic_caps_ext & IEEE80211_CEXT_PBCC).  This is only done for 2GHz
  channels.
- Teach ifconfig(8) to print ieee80211.ic_caps_ext upon "list caps" command.