ssh: Add 'remote-inferior'.

* guix/inferior.scm (<inferior>)[close]: New field.
(port->inferior): New procedure.
(open-inferior): Rewrite in terms of 'port->inferior'.
(close-inferior): Honor INFERIOR's 'close' field.
(inferior-eval-with-store): Add FIXME comment.
* guix/ssh.scm (remote-inferior): New procedure.