Introduce DUMP_BEACON command

Introduce DUMP_BEACON command

Occasionally, external applications require information about AP
configurations and capabilities. One potentially useful source for this
is the Beacon frame content. To support this need, introduce a new
control interface command: DUMP_BEACON. This return a hexdump of the
Beacon frame template, i.e., IEEE 802.11 frame header and frame body
with the TIM element missing since it is added by the driver and some of
the fields like the Timestamp field left to all zeros since they will be
filled in by the driver/hardware.

This can be fetched with hostapd_cli:

hostapd_cli -i wlxxx raw DUMP_BEACON
80000000ffffffffffff...
7f080400000200000040dd180050f2020101010003a4000027a4000042435e0062322f00

Signed-off-by: Marek Puzyniak <marek.puzyniak@holisticon.pl>