hostapd: auto-select secondary channel

Select a secondary channel according to regulatory class and ambient
noise constraints, as measured by the number of APs on the primary
channel.
If the user wants HT40 operation, try to select a secondary channel
where the level of noise is still minimal. If such one cannot be found,
we prefer to operate as an HT20 AP. The secondary channel selected also
has the minimum number of APs between all allowed secondary channels.

HT40 on 2.4Ghz is disallowed outright, since we can't fulfill all
regulatory constraints to operate in this mode.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
hostapd: add support for ACS whitelist and blacklist

Allow certain channels to be marked as forbidden for automatic channel
selection (ACS) via a blacklist. Also allow channels to be preferred
exclusively via a whitelist.

Both blacklist and whitelist can be configured via the hostapd.conf
file. Document these options in the sample hostapd.conf file given.

Signed-off-by: Arik Nemtsov <arik@wizery.com>