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/hdb-ldap.c

@@ -2097,18 +2097,18 @@ fini(void *ctx)
 
 struct hdb_method hdb_ldap_interface = {
     HDB_INTERFACE_VERSION,
-    0 /*is_file_based*/, 0 /*can_taste*/,
     init,
     fini,
+    0 /*is_file_based*/, 0 /*can_taste*/,
     "ldap",
     hdb_ldap_create
 };
 
 struct hdb_method hdb_ldapi_interface = {
     HDB_INTERFACE_VERSION,
-    0 /*is_file_based*/, 0 /*can_taste*/,
     init,
     fini,
+    0 /*is_file_based*/, 0 /*can_taste*/,
     "ldapi",
     hdb_ldapi_create
 };