Currently, if you try to run a repl command (for example, "M-x
guix-installed-packages") in a second Emacs instance, you'll get an
unfriendly error.  That's because `guix-default-port' is busy already
(by another Guix REPL), so you either have to change it manually or use
(setq guix-use-guile-server nil).  So with the attached patch, a socket
file with a generated name will be used instead of a port, which allows
you to run as many Emacs instances with Guix REPLs as you want.

Many thanks to Florian for the great idea!

There is one small thing though: Guile does not remove socket file after
exiting from "guile --listen=/tmp/foo" so these dead sockets will stay in
/tmp dir.  As there is no `comint-exit-hook' or alike, I don't see how a
socket file can be removed after the REPL is killed.