kadm5/kadmin: Add read-only mode

Now we can have read-only kadmind instances.

lib/kadm5/create_c.c

@@ -64,7 +64,7 @@ kadm5_c_create_principal(void *server_handle,
     if (n_ks_tuple > 0)
 	return KADM5_KS_TUPLE_NOSUPP;
 
-    ret = _kadm5_connect(server_handle);
+    ret = _kadm5_connect(server_handle, 1 /* want_write */);
     if (ret)
 	return ret;