pull: Add a compilation progress report.

* guix/build/pull.scm (report-build-progress): New procedure.
  (p-for-each): Add #:progress parameter.
  [loop]: Keep track of the number of completed processes.  Tail-call
  PROGRESS at each loop iteration.
  (build-guix): Add #:debug-port parameter.  Use it for verbose
  messages.  Change 'tar' flags to 'xf'.  Around 'compile-file' call,
  bind CURRENT-WARNING-PORT to DEBUG-PORT.
* guix/scripts/pull.scm (unpack): Add #:verbose? parameter.
  [builder]: Pass #:debug-port to 'build-guix'.
  (guix-pull): Leave CURRENT-BUILD-OUTPUT-PORT unchanged.  Pass
  #:verbose? to 'unpack'.