Suggested by Florian Paul Schmidt.
* emacs/guix-backend.el (guix-default-port): Remove.
(guix-repl-socket-file-name-function, guix-repl-current-socket): New
variables.
(guix-repl-socket-file-name, guix-repl-delete-socket-maybe): New
procedures.
(guix-get-guile-program): Take socket as an optional argument.
(guix-start-repl-maybe): Adjust accordingly.