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/test_pw_quality.c

@@ -31,10 +31,14 @@
  * SUCH DAMAGE.
  */
 
-#include "kadm5_locl.h"
+#include <string.h>
+#include <stdlib.h>
 #include <getarg.h>
 
-RCSID("$Id$");
+#include <roken.h>
+#include <krb5.h>
+
+#include "admin.h"
 
 static int version_flag;
 static int help_flag;