sys.kern.pdeathsig.signal_delivered_ptrace: fix debugger detach

sys.kern.pdeathsig.signal_delivered_ptrace: fix debugger detach

ptrace(PT_DETACH) requires stopped debuggee, otherwise it fails.  When
the call fails, the C process is left as debuggee of the process D,
and might be killed too early if process D exits occurs fast enough.

Since pipes are not closed in the forked children, this resulted in
the test hanging, since no write occured from C to wake A.

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