In net80211, enable RTS for frames above a particular size (currently 512

In net80211, enable RTS for frames above a particular size (currently 512
bytes). This is what other OS have been doing for years. In our stack this
feature was present but disabled at compile-time by an #ifdef.

This is a low risk change because drivers were already required to use RTS
whenever the AP set the USE_PROTECTION flag in ERP elements of its beacons.

This change allows for reasonable throughput on loaded 11g networks
whereas before they were practically unusable.

tests and ok phessler@