network: set broadcast address on request

Previously, the broadcast address was set to a Address object in
address_section_verify() (or address_acquire()). But, for wireguard
interfaces, we do not use the broadcast address. The .network file may
be assigned to multiple interfaces, hence, we cannot determine if we
should set the broadcast address in address_section_verify().

This makes the broadcast address set in link_request_address().
Then, we set the broadcast address only when we need it.