dtrace: prevent forked child from running after an error condition

The pid/killonerror test uses an invalid probe specifier to verify that
the child process is killed. It occasionally fails because the "date"
command is allowed to run long enough to print the date. This is harmless
in this case, but is clearly not ideal.

When the dt_proc_control thread is about to exit, and the dtrace command
forked the child, do not make the child runnable.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D40976