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.h

@@ -307,11 +307,9 @@ typedef struct HDB {
 #define HDB_INTERFACE_VERSION	11
 
 struct hdb_method {
-    int			version;
+    HEIM_PLUGIN_FTABLE_COMMON_ELEMENTS(krb5_context);
     unsigned int	is_file_based:1;
     unsigned int	can_taste:1;
-    krb5_error_code	(*init)(krb5_context, void **);
-    void		(*fini)(void *);
     const char *prefix;
     krb5_error_code (*create)(krb5_context, HDB **, const char *filename);
 };