offload: Prevent the '.drv' and build result from being GC'd.

Before that, there was a small time window during which the GC could
wipe the .drv (before 'guix build' has been called), or the build
result (before 'retrieve-files' has started.)

* guix/scripts/offload.scm (remote-pipe): Add #:quote? parameter and
  honor it.
  (%gc-root-file): New variable.
  (register-gc-root, remove-gc-root): New procedures.
  (offload): Adjust comment.  Run 'guix build' with '-r %GC-ROOT-FILE'.
  (transfer-and-offload): Call 'register-gc-root' before
  sending (derivation-file-name DRV).  Call 'remove-gc-root' after the
  call to 'offload' or 'retrieve-files'.
  (send-files): Call 'remote-pipe' with #:quote? #f.
  (retrieve-files): Likewise.

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