hostapd: add default_disabled option to the supplicant

hostapd: add default_disabled option to the supplicant

With the default configuration we generate, the supplicant starts
scanning and tries to connect to any open network when the interface
is enabled.

In some cases it can be desirable to prevent the supplicant from
scanning by itself. For example, if on the same radio an AP is
configured and an unconfigured STA is added (to be configured with
WPS), the AP might not be able to beacon until the STA stops
scanning.

In such a case, the STA configuration can still be required to set
specific settings (e.g. multi_ap_backhaul_sta) so it can't be set to
"disabled" in uci (because that would prevent the supplicant from
being run at all). The alternative is to add the "disabled" parameter
to the default network block in the supplicant configuration.

This patch adds a "default_disabled" setting in UCI which, when set,
adds the "disabled" parameter to the supplicant default network block.

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>