hostapd: make wnm_sleep_mode_no_keys configurable

hostapd: make wnm_sleep_mode_no_keys configurable

In the aftermath of the KRACK attacks, hostapd gained an AP-side workaround
against WNM-Sleep Mode GTK/IGTK reinstallation attacks. WNM Sleep Mode is not
enabled by default on OpenWrt, but it is configurable through the option
wnm_sleep_mode. Thus, make the AP-side workaround configurable as well by
exposing the option wnm_sleep_mode_no_keys. If you use the option
wpa_disable_eapol_key_retries and have wnm_sleep_mode enabled, you might
consider using this workaround.

Signed-off-by: Timo Sigurdsson <public_timo.s@silentcreek.de>