network/address: acquire address in address_process_request()

network/address: acquire address in address_process_request()

Previously, if an [Address] section is configured with a null address,
e.g. Address=0.0.0.0/24, then we acquired a free address in
link_request_address().

With this commit, we queue a request with the null address as is, and
acquire a free address later in address_process_request(). Similary,
now IPv4ACD daemon is configured in address_process_request().

With this change, we can make the address acquisition depend on other
conditions, e.g. if the persistent storage is ready or not.