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/test_namespace.c

@@ -243,12 +243,15 @@ 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,
     "test",
     hdb_test_create
 #else
     .version = HDB_INTERFACE_VERSION,
+    .is_file_based = 1,
+    .can_taste = 1,
     .init = hdb_test_init,
     .fini = hdb_test_fini,
     .prefix = "test",