stop_all_proc(): skip traced or signal-stoped processes

Since thread_single(SINGLE_ALLPROC) ignores them since 9241ebc796c,
and there is not much we can do for the debugger-controlled process.

Noted by: olce
Reviewed by: markj, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D44638