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