Introduce macro for common plugin structure elements

Heimdal's HDB plugin interface, and hence Samba's KDC that depends upon
it, doesn't work on 32-bit builds due to structure fields being arranged
in the wrong order. This problem presents itself in the form of
segmentation faults on 32-bit systems, but goes unnoticed on 64-bit
builds thanks to extra structure padding absorbing the errant fields.

This commit reorders the HDB plugin structure fields to prevent crashes
and introduces a common macro to ensure every plugin presents a
consistent interface.

Samba BUG: https://bugzilla.samba.org/show_bug.cgi?id=15110

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>

lib/hdb/test_namespace.c

@@ -243,17 +243,17 @@ struct hdb_method hdb_test =
 #ifdef WIN32
     /* Not c99 */
     HDB_INTERFACE_VERSION,
-    1 /*is_file_based*/, 1 /*can_taste*/,
     hdb_test_init,
     hdb_test_fini,
+    1 /*is_file_based*/, 1 /*can_taste*/,
     "test",
     hdb_test_create
 #else
-    .version = HDB_INTERFACE_VERSION,
-    .is_file_based = 1,
-    .can_taste = 1,
+    .minor_version = HDB_INTERFACE_VERSION,
     .init = hdb_test_init,
     .fini = hdb_test_fini,
+    .is_file_based = 1,
+    .can_taste = 1,
     .prefix = "test",
     .create = hdb_test_create
 #endif