shutdown: tweak kproc/kthread shutdown check

This is to handle the case where the system has not panicked but the
debugger is active, where we still can't wait for thread termination.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42683

(cherry picked from commit 960612a19f009df602a4cb008fa90a45a6e869bb)