networkd: Add hashmap to store prefixes and associated link

Add a hashmap to the Manager struct that stores the association
between an IPv6 prefix and the network Link it is assigned to.
This is added in order to keep assigning the same prefixes with
the same links even though they are delegated at different times
or by different DHCPv6 clients.

src/network/networkd-manager.h

@@ -58,6 +58,7 @@ struct Manager {
         Hashmap *links;
         Hashmap *netdevs;
         Hashmap *networks_by_name;
+        Hashmap *dhcp6_prefixes;
         LIST_HEAD(Network, networks);
         LIST_HEAD(AddressPool, address_pools);
 
@@ -109,5 +110,10 @@ Link* manager_find_uplink(Manager *m, Link *exclude);
 int manager_set_hostname(Manager *m, const char *hostname);
 int manager_set_timezone(Manager *m, const char *timezone);
 
+Link *manager_dhcp6_prefix_get(Manager *m, struct in6_addr *addr);
+int manager_dhcp6_prefix_add(Manager *m, struct in6_addr *addr, Link *link);
+int manager_dhcp6_prefix_remove(Manager *m, struct in6_addr *addr);
+int manager_dhcp6_prefix_remove_all(Manager *m, Link *link);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(Manager*, manager_free);
 #define _cleanup_manager_free_ _cleanup_(manager_freep)