shutdown: tweak kproc/kthread shutdown check

[mhorne]

Summary

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.

[mhorne]

sys/kern/kern_shutdown.c

1017–1025

To be clear, the impact of the change is that we don't execute this block and don't print anything to console. The underlying call to _sleep() returns immediately with no error when the scheduler is stopped. So it is mostly a change for "correctness".