kqueue: on process exit, force-clear its registered signal events

Normally, process already has all its kqueue fds destroyed at the moment
p_klist is detached in exit flow. But, if the process was created with
rfork(2) with shared file descriptors, its signal knotes can survive.
Then, knlist_detach() does not destroy non-empty knlist. Later, when
owning kqueue is closed, we access freed (or rather, reused, because
struct proc is type-stable) memory by referencing p->p_klist from such
knote.

Handle this situation by deleting all knotes hanging from p_klist.

PR: 275286
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42745