hdb: Remove default HDB backend footgun

Do not allow a change in build configuration time default HDB backend
selection cause existing default HDBs to not be possible to open.

Otherwise such a change will cause a KDC configured to use the default
HDB (i.e., without setting it in the "database" stanza in the "[kdc]"
section of krb5.conf) to not start.

lib/hdb/hdb.h

@@ -303,6 +303,8 @@ typedef struct HDB {
 
 struct hdb_method {
     int			version;
+    unsigned int	is_file_based:1;
+    unsigned int	can_taste:1;
     krb5_error_code	(*init)(krb5_context, void **);
     void		(*fini)(void *);
     const char *prefix;