whether to allow parsing
IPv6 addresses. If both NM_IP_ROUTING_RULE_AS_STRING_FLAGS_AF_INET and
NM_IP_ROUTING_RULE_AS_STRING_FLAGS_AF_INET6 are unset, it's the same
as setting them both.
no flags selected.
if set, ensure that the rule verfies or fail.
whether to allow parsing IPv4 addresses.