validation: warn if a priority is not set for a routing-policy

The iproute2 guidance says "Each rule should have an explicitly set
unique priority value", and further warns:

    For historical reasons ip rule add does not require any priority
    value and allows the priority value to be non-unique. If the user
    had not supplied a priority value then one was assigned by the
    kernel. If the user requested creating a rule with a priority value
    which already existed then the kernel did not reject the request and
    added the new rule before all old rules of the same priority. This
    is a mistake in the current design, nothing more. It should be fixed
    by the time you read this so please do not rely on this feature. You
    should always use explicit priorities when creating rules.

So let's inspect each routing-policy entry and emit a warning if a
priority has not been specified.

ref. http://www.policyrouting.org/iproute2.doc.html#ss9.6.1

Author: drafnel

src/validation.c

@@ -523,6 +523,8 @@ adopt_and_validate_vrf_routes(__unused const NetplanParser *npp, GHashTable *net
         if (nd->ip_rules) {
             for (size_t i = 0; i < nd->ip_rules->len; i++) {
                 NetplanIPRule* r = g_array_index(nd->ip_rules, NetplanIPRule*, i);
+                if (r->priority == NETPLAN_IP_RULE_PRIO_UNSPEC)
+                    g_warning("%s: No priority specified for routing-policy %zu", nd->id, i);
                 if (r->table == nd->vrf_table) {
                     g_debug("%s: Ignoring redundant routing-policy table %d (matches VRF table)", nd->id, r->table);
                     continue;