wifi: Add new options to enable RTS/CTS protection
- Add an attribute (
ns3::WifiDefaultProtectionManager::SingleRtsPerTxop
) to use RTS/CTS protection no more than once in a TXOP, regardless of the destination of the data frame - Add an attribute (
ns3::WifiRemoteStationManager::RtsCtsTxDurationThresh
) to enable RTS/CTS protection based on the TX duration of a PPDU (in addition to the existing length-based threshold -- if either of the two thresholds is exceeded, RTS is enabled)