WNM: Configurable BSS Max Idle Period management on AP

WNM: Configurable BSS Max Idle Period management on AP

Allow AP's behavior for BSS Max Idle Period management to be configured.
Previously, this was automatically enabled for all CONFIG_WNM_AP=y
builds. This can now be changed with the new hostapd configuration
parameter bss_max_idle:
0 = BSS Max Idle Period management disabled
1 = BSS Max Idle Period management enabled
    (default and the previous behavior)
2 = BSS Max Idle Period management enabled with requirement for
    protected keep-alive frames

Signed-off-by: Jouni Malinen <quic_jouni@quicinc.com>