Add a few new fields to the RX vendor radiotap header:

* a flags field that lets me know what's going on;
* the hardware ratecode, unmolested by conversion to a bitrate;
* the HAL rs_flags field, useful for debugging;
* specifically mark aggregate sub-frames.

This stuff sorely needs tidying up - it's missing some important
stuff (eg numdelims) and it would be nice to put the flags at the
beginning rather than at the end.

Tested:

* AR9380, STA mode, 2x2 HT40, monitoring RSSI and EVM values