tdb2: add TDB_VERSION1 flag.

This will be set for old TDBs; we can start distinguishing paths based
on it now.

ccan/tdb2/tdb1_open.c

@@ -180,7 +180,7 @@ struct tdb1_context *tdb1_open_ex(const char *name, int hash_size, int tdb1_flag
 	tdb->fd = -1;
 	tdb->name = NULL;
 	tdb->map_ptr = NULL;
-	tdb->flags = tdb1_flags;
+	tdb->flags = tdb1_flags|TDB_VERSION1;
 	tdb->open_flags = open_flags;
 	if (log_ctx) {
 		tdb->log_fn = log_ctx->log_fn;

ccan/tdb2/tdb2.h

@@ -86,6 +86,7 @@ struct tdb_context *tdb_open(const char *name, int tdb_flags,
 #define TDB_SEQNUM   128 /* maintain a sequence number */
 #define TDB_ALLOW_NESTING   256 /* fake nested transactions */
 #define TDB_RDONLY   512 /* implied by O_RDONLY */
+#define TDB_VERSION1  1024 /* create/open an old style TDB */
 
 /**
  * tdb_close - close and free a tdb.