Change struct empfile callback onresize() to return void

ef_open() handles onresize() failing incorrectly.  Instead of fixing
that, drop the failure mode.  It's not really used: unit_onresize()
fails only when used incorrectly.  It isn't.  If it ever is, ignoring
the failure is safe.

include/file.h

@@ -79,9 +79,8 @@ struct empfile {
     void (*prewrite)(int id, void *old, void *elt);
     /*
      * Called after table size changed, with file type as argument.
-     * Return -1 and set errno to make the operation fail.
      */
-    int (*onresize)(int type);
+    void (*onresize)(int type);
 };
 
 struct emptypedstr {