Avoid a second check for 0 in file's _lock().
Fix uninit warning in boro_k_delete().
Do not call _headr() from boro_get_from_del_tree() if nothing changed.
Add the boro_node pointer to boro_set_left_p() and boro_set_right_p().

This avoids some redundant calls to boro_node().
Added flush and dispose code to private functions.