New API: debug-upload (not a formal part of the API/ABI).

Allow arbitrary files to be uploaded into the appliance, but
only when --enable-debug-command is enabled.  This lets you
run shell scripts, like this:

  ><fs> debug-upload -<<EOF /tmp/script.sh 0700
  #!/bin/sh -
  # ...
  EOF
  ><fs> debug sh "/tmp/script.sh"