Memory safety bugs in kqueue copy-on-fork implementation

When copying knotes from a parent kqueue to a child, the copy code did not correctly exclude marker knotes (used internally to track list traversal position) before marking them as in-flux and releasing the kqueue lock. If another thread freed a marker while the lock was dropped, the subsequent in-flux decrement operated on freed memory.

An unprivileged local user may be able to exploit these races to escalate privileges.