ssh: 'send-files' displays a progress bar.

* guix/store.scm (export-paths): Add #:start, #:progress, and #:finish
parameters and honor them.
* guix/ssh.scm (prepare-to-send, notify-transfer-progress)
(notify-transfer-completion): New procedures.
(send-files): Pass #:start, #:progress, and #:finish to 'export-paths'.