Add a config parameter to exclude DFS channels from ACS

Add a config parameter to exclude DFS channels from ACS

The new acs_exclude_dfs=1 parameter can be used to request hostapd to
exclude all DFS channels from ACS consideration. This is mainly of use
for cases where the driver supports DFS channels, but for some reason a
non-DFS channel is desired when using automatic channel selection.
Previously, the chanlist parameter could have been used for this, but
that required listing all the acceptable channels. The new parameter
allows this to be done without such a list.

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