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/krb5/krb5.h

@@ -873,14 +873,16 @@ typedef krb5_error_code
 (KRB5_CALLCONV * krb5_sendto_ctx_func)(krb5_context, krb5_sendto_ctx, void *,
 				       const krb5_data *, int *);
 
-struct krb5_plugin;
 enum krb5_plugin_type {
     PLUGIN_TYPE_DATA = 1,
-    PLUGIN_TYPE_FUNC
+    PLUGIN_TYPE_FUNC /* no longer supported */
 };
 
 #define KRB5_PLUGIN_INVOKE_ALL  1
 
+typedef uintptr_t
+(KRB5_CALLCONV *krb5_get_instance_func_t)(const char *);
+
 struct credentials; /* this is to keep the compiler happy */
 struct getargs;
 struct sockaddr;