Switch scp from using pipes to a socketpair for communication with

Switch scp from using pipes to a socketpair for communication with
it's ssh sub-processes.  We no longer need to reserve two descriptors
to ensure that we don't end up using fd 0-2 unexpectedly, that is
handled by sanitise_stdfd() in main().
Based on an original diff from djm@.  OK deraadt@ djm@