interfaces: store interface broadcast addresses

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
mndp: use directed broadcasts for MNDP discovery requests

Makes discovery more reliable on multihomed systems.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
mactelnet: add support for dbclient as SSH client

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>