sys.kern.pdeathsig.signal_delivered_ptrace: fix startup.

Inform D that C executed procctl(PROC_PDEATHSIG_CTL). Otherwise D
might allow B to exit before C is set up to receive a signal on the
parent exit. In this case, C waits forever for the signal and test
hangs.

PR: 237657
Reported and tested by: lwhsu
Sponsored by: The FreeBSD Foundation
MFC after: 1 week