process_pipe: warn hackers off using it for bidirectional pipes

process_pipe: warn hackers off using it for bidirectional pipes

While most uses of ->DESTROY happens in a predictable order in
long-lived daemons, process teardown on exit is chaotic and not
subject to ordering guarantees, so we must keep both ends of a
`git cat-file --batch*' pipe at the same level in the object
hierarchy.

Drop an old Carp import while I'm in the area.