make creation of trash folder independent from -C option

the trash is not a box which is synced, but a "byproduct" of
manipulating synced boxes, so it makes no sense to bind it to the same
option.

src/drv_maildir.c

@@ -1029,7 +1029,7 @@ maildir_store_msg( store_t *gctx, msg_data_t *data, int to_trash,
 			cb( DRV_BOX_BAD, 0, aux );
 			return;
 		}
-		if ((ret = maildir_validate( gctx->conf->path, gctx->conf->trash, gctx->opts & OPEN_CREATE, ctx )) != DRV_OK) {
+		if ((ret = maildir_validate( gctx->conf->path, gctx->conf->trash, 1, ctx )) != DRV_OK) {
 			free( data->data );
 			cb( ret, 0, aux );
 			return;