Allow wpa_supplicant AP mode to configure Beacon interval

Allow wpa_supplicant AP mode to configure Beacon interval

beacon_int (in TU) can now be used to configure Beacon interval for AP
mode operations (including P2P GO) in wpa_supplicant. This can be set
either in a network block or as a global parameter in the configuration
file (or with "SET beacon_int <value>" control interface command) to
apply for all networks that do not include the beacon_int parameter to
override the default.

In addition, this commits extends the dtim_period parameter to be
available as a global parameter to set the default value. dtim_period is
now stored in the configuration file, too, if it was set.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>