krb5, kadm5: refactor plugin API

Refactor plugin framework to use a single list of loaded plugins; add a new
plugin API where DSOs export a load function that can declare dependencies and
export multiple plugins; refactor kadm5 hook API to use krb5 plugin framework.

More information in krb5-plugin(7).

lib/kadm5/version-script.map

@@ -23,6 +23,7 @@ HEIMDAL_KAMD5_SERVER_1.0 {
 		kadm5_free_name_list;
 		kadm5_free_policy_ent;
 		kadm5_free_principal_ent;
+		kadm5_get_instance;
 		kadm5_get_policy;
 		kadm5_get_policies;
 		kadm5_get_principal;