hostapd: add legacy_rates option to disable 802.11b data rates.

Setting legacy_rates to 0 disables 802.11b data rates.
Setting legacy_rates to 1 enables 802.11b data rates. (Default)

The basic_rate option and supported_rates option are filtered based on this.

The rationale for the change, stronger now than in 2014, can be found in:

https://mentor.ieee.org/802.11/dcn/14/11-14-0099-00-000m-renewing-2-4ghz-band.pptx



The balance of equities between compatibility with b clients and the
detriment to the 2.4 GHz ecosystem as a whole strongly favors disabling b
rates by default.

Signed-off-by: Nick Lowe <nick.lowe@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cleanup, defaults change]