Support RX flags, MCS and the vendor namespace, and fix Rate.

Support RX flags, MCS and the vendor namespace, and fix Rate.

Based on patches from an anonymous donor, support the radiotap RX flags and
MCS fields, and the vendor namespace, and, if Channel and XChannel are
both present, use XChannel, not Channel.

Do not try to look up a rate for an MCS value from the Rate field; you
cannot map an MCS value to a rate without also knowing the channel width
and guard interval length.